Why aren't the zombies decomposed after all this time?

You're, perhaps, assuming that all the walkers are the same age. With each new death (and a walker doesn't have to be involved, because everyone has the virus in them), a new walker arises. You would expect that the walker population would eventually shrink and be gone, which is a great theory postulated by Andrew Martin. That is actually a logical conclusion to the series, and maybe one the writers haven't thought of yet. But for now, it's very possible that the walkers you're seeing aren't the same ones that were around in the beginning.

TL;DR:

Magic? To keep the show on air as long as possible? The virus somehow keeps them alive? All viable options.

Long Answer:

To shamefully steal paraphrase from the answers given on the same topic over at the Sci Fi Stack Exchange:

As most people are aware... Georgia (and most of the Southeastern USA) gets extremely hot and humid throughout the summer months. A human body in that environment would decompose in under a week...

We have to assume that the "zombie virus" somehow prevents decomposition (or drastically reduces its speed). Decomposition results from little critters (primarily micro-organisms) digesting the organic material. They cannot do this to living things, because we have immune systems which thwart those creatures before they start.

Additionally, to quote from the accepted answer on that site:

In most cases the answer is the rather unsatisfying "its magic." Whatever mystic force re-animates the body of the zombie also halts the decomposition process, the same as it does for other undead like vampires.

To add my own thoughts here, I think ultimately the real answer is - if they zombies were decomposed, the most-watched drama in US TV history* would be over.

As this article shows, it does appear that the makers of the show have deliberately decomposed the zombies season by season. They also mention it in an interview to the Wall Street Journal, saying:

"The walkers who came around patient zero, which is over two years, their systems aren't working quite so well. They're more and more decayed. That's what we're seeing [in Season 5]," Hurd [the executive producer] recently told The Wall Street Journal.

Greg Nicotero, the showrunner, added:

"We're a year and a half into the zombie apocalypse, so imagine what a pumpkin looks like after just 2 weeks in the sun...[laughs] we wanted to make these things look as putrefied and disgusting as possible," he said.

Perhaps t hey are simply building up to an eventual show-ending season where the zombie threat is finally eliminated and society can try and rebuild? Who knows.

I would say the zombies will magically stay around until the show's ratings decline and then a storyline might be made out of it.

_{* Okay, I know there's a difference between "most-watched episode" ever and "most-watched series" ever, but I like the punchiness of my statement :)}

This is a short one but might do the trick: I distinctly remember Milton Mamet saying the creature's metabolism is a lot slower than a human's, which explained why they can go for so long without food. This directly implies that at some level they're still biologically alive.

I don't remember the exact episode; I think it was when he and Michonne were discussing her captive walkers, and how they lost their "appetite" with their jaws. Being "somewhat" alive also explains why they appear to be "somewhat" rotting.

Note however that while the flesh should decompose, the skeleton is another story. If the condition of the skull is indicative of the state of the walker's skeletons, then one would ony have to push them to literally break them apart. I think the ease of skull destruction cannot be explained without more details about the condition, so we know how it affects the bones. Or it could be explained by saying "the characters need to kill the walkers quickly".

I have never been a big fan of the idea that zombies can be explained scientifically in any canon, and The Walking Dead is no exception. In the pilot, as soon as Rick wakes up and is wandering around, we already see several zombies in a state of advanced decomp. One is crawling on the ground with an exposed rib cage, for instance.

No metabolism would be possible. No blood is flowing through their veins... where is the oxygen coming from? How are muscles contracting? Even the go-to tropes for science fiction don't work here... nanobots would have a hell of a time pushing some fat 200lb corpse around (and balancing it with bipedal ambulation!).

The only reasonable explanation is the supernatural. Some demonic or spiritual force animates the cadavers, and it is moving them with what might as well be telekinesis.

The reason that head/brain damage de-animates can only be guessed at. If these are possessed corpses, then that may be the key that allows whatever force it is to animate them... once gone, it can't animate a headless corpse anymore than it could animate a refrigerator.

The characters in the show tend to favor more rational explanations, but I think that they are mistaken (as people are in real life about many things).

Super-Soldier Serum

This is not explained in the show or the comics. I don't know if the authors have even thought about it - many times, this is simply thought of as a conceit of the genre.

However, there's no actual reason why things have to decay.

Decay is caused by various kinds of microbes, as well as oxygen and sunlight damage

Bacteria and fungi decompose corpses by digesting them looking for nutrients - similarly, corpses are damaged by air movement, oxidative stress, and the impact of heat and cold on their tissue fibres.

If an organism (say, the Z virus) is able to kill or hinder attempts by microbes to digest the host body, and form barriers against various environmental effects that would wear down or wear away the body, as well as deal with the various negative effects caused by the host body's functions mostly ceasing, then there's no reason the body would continue to decay.

The 'surface decay' may be the only part of the body rotting.

But what about zombies walking around and stuff for years after they've died? Where is the energy coming from?

The chemical energy contained in the body is enough to sustain a degree of activity with no energy-influx. However, if these 'walkers' have been shambling around for a year, with no food, they should be skin and bones and also dead. If they aren't, something like drastic improvements to the muscle system of the body, cellular fusion, photosynthesis, etc, must be occurring.

Could a virus do this?

Maybe. It would be extremely unlikely for a set of micro-organisms to be able to do this kind of thing, although it's not impossible - with luck, genius, and skill, a modern-day biowarfare lab might be able to produce something like this.

It's far more likely, though, that if something like this did exist in the real world it would be the product of a far more advanced civilization. A meteor or a derelict spaceship crashed on earth with self-modifying micromachines or micro-organisms on board, that were able to make themselves work with earth biology. The 'zombie' effect is likely accidental - whatever they were designed to do, confronted with a wildly different type of creature than they were designed for, their effect is different than it was designed to be.

That the scientist at the CDC said that he couldn't even classify the micro-organisms as bacteria, fungi, or viral is evidence for this kind of quantum leap in nanowarfare.

That the micro-organisms activate once the brain function ceases indicates it may be an alien medical tool desperately attempting to save the 'patient', with the zombification being a side-effect of the micro-organisms/machines being designed for something that while similar enough to earth life to attempt to interface with it, is wildly different enough to produce such results.

They don't stop decomposing, but they do so rather slowly. In The Walking Dead, unlike the book World War Z, zombies steadily decompose, albeit rather slowly. In the book World War Z, the process still takes place, but usually at a far slower rate (the exception being the rare cases in which the zombie is in an extremely humid, hot environment, like a jungle; in this scenario, the zombie will decompose very quickly).

Getting back to TWD, the fact that the zombies decompose is particularly noticeable in the cases of zombies who have spent some time in the water. Their bodies become bloated and water logged, and their tissue becomes quite fragile:

The production team has also explicitly stated that as the seasons progress, they have tried to make the zombies appear more and more decomposed and emaciated. I have to admit that, in my opinion, they have been extremely inconsistent in this regard, but they claim to have taken it into consideration, and tried to reflect the fact that time is passing and the zombies are becoming increasingly decrepit.

It was most apparent in season 2, in which you really can see how skinny most of the zombies are. The casting director specifically selected people who were extraordinarily thin, and if I recall correctly, they had a lot of success in hiring marathon runners for exactly this reason.

Robert Kirkman has also said that as time goes by, the zombies lose much of their mobility, speed, and strength, as well as their already-meager amount of intelligence. This is actually how he explained the fact that in season one, and only season one (in fact, only the first two episodes of that season), zombies do things that seem to be beyond their capabilities - they almost run in some scenes, they use rocks to break windows, Morgan's zombie wife tries to turn a doorknob, and one little girl zombie stops to pick up her teddy bear. According to Kirkman, all of this is explained by the fact that the zombies are much more intact and undecayed, relative to the zombies of later seasons.

From a Reddit Ask Me Anything with Robert Kirkman:

Q: "...In the beginning of the show we saw walkers do things like using a rock to help bash the doors in or turning a door knob, is there a reason we've stopped seeing them do that?"

A: "Older zombies are less together and capable or doing things like that. Fresher zombies, which there were more of in season one, are able to do more than older, more rotted zombies."

If we're talking about TWD zombie bones as compared to real human bones, it seems clear that, although no one really talks about it, the zombies' bones, especially their skulls, are much less robust than our own.

As I said, no one seems to mention it on the show, but we can see it quite clearly. People routinely crush zombie skulls by stomping on them, hitting them with fairly flimsy objects, etc, and the most popular method of killing zombies is now the classic stab-in-the-head.

While I don't recommend that you try this at home, I can assure you that stabbing someone in the face won't usually result in the knife piercing their brain cavity. The skull is surprisingly thick, robust, and extremely tough. It is very difficult to penetrate it, especially with something like a knife or screwdriver.

The shape of the human skull - almost spherical in the upper portion - is such that a thrust with a knife is almost certain to be deflected and slide off to one side, which might inflict quite a bit of damage to the scalp and/or face, but the skull itself will suffer little or no damage whatsoever. Even if you managed to strike the skull perpendicular to the forehead, for instance, the knife might very well stop short, creating a possibility that you will injure your own wrist, or even worse, that your hand will slip forward and be sliced open by the blade.

As for stomping on a head, this might cause a concussion in a human victim, but it probably wouldn't crush the skull outright. And yet we see, again and again, zombie skulls popping open like grapes when the slightest amount of pressure is applied.

Moving on to blunt objects, such as baseball bats, rocks, clubs, and so on, again, the injuries inflicted upon zombies are far more serious than would be the case if the target was a living person. Yes, of course a powerful blow from a bat would fracture a human skull, but fractures aren't enough to kill a zombie. You need to destroy the brain, not merely create a few hairline fractures in the bone around it. Once again, attacks which would only hurt a person are consistently shown to kill zombies.

In season 3, we saw a zombie in the prison, whose hands were cuffed behind his back, snap his own arm in half with very little effort. This too would be almost impossible for a living human being to do. And in the season 5 finale, Rick was able to slowly shove a gun barrel through the roof of a zombie's mouth and into the cranial cavity. Also something that you couldn't do to a human without a tremendous amount of force.

We've also seen two zombies being killed by holding a relatively thin branch perpendicular to their heads, at about mouth-level, and pushing with very little force. Branches less than an inch and a half in diameter are not as strong as a human skull. The branch should break before the skull does.

And in season 4, when zombies began to fall through the ceiling of a big box store, a drop of about 15-20 feet, they literally popped when they hit the ground, almost like water balloons. A person who fell the same distance would probably break some bones, but they certainly wouldn't explode.

All in all, we are never told that zombie skeletons are far less robust than human skeletons, but all the evidence points in exactly this direction. Zombie bones are much more brittle and prone to breaking than human bones.

In a recent interview, Fear the Walking Dead showrunner Dave Erickson said that on that show, zombies will be more difficult to kill than they are on the original The Walking Dead series, because Fear the Walking Dead takes place at the beginning of the outbreak, whereas TWD is now at least a couple of years into it. As a result, zombie skulls on FTWD are stronger and harder to penetrate than has been the case on TWD.

Erickson said one of Kirkman's notes for Fear was that the zombies be different than how they were first seen on the original show. With the early onset of the outbreak, skulls will still be hard and the point, Erickson said, is to see a character trying to stab a walker in the head and not be able to penetrate the skull. "The point was it's hard physically to kill somebody," he said.

In fact, we've already seen this happen, on the second episode of the new series.

OK for one thing all zombies must be alive to some extent, because you have to destroy the brain to kill them right? If you think about it that that means the brain is working or working to an extent. For them to move, the brain must be functional enough for the body to use basic functions provided by the nervous, muscle, and skeletal systems. That's never blatantly stated but if you use your head, that's what allows the human body to move. As for the decay, I've wondered that myself since the first time I had ever heard of zombies. Probably, decomposition happens much slower in the winter than the summer, and like I mentioned above the zombie has to be alive somewhat; enough for the brain to do what it needs to do to keep the walkers walking, so they are probably also alive enough they don't decompose as fast as actual corpses. And another thing, we really don't know what time it is in the zombie apocalypse now do we? Sometimes the show jumps months ahead without mentioning it, sometimes its the next day, so honestly not as much time might be passing as you think.