Why didn't Harry himself drink Polyjuice Potion instead of all others in Deathly Hallows Part 1?

There are a few reasons why they all chose to take the form of Harry, but these are entirely explained in the books as opposed to the films.

I'll start with my own speculation, before moving on to evidence from the book. I believe if they disguised him as, for example, his cousin Dudley, it would look especially suspicious. Imagine if Voldemort and the Death Eaters had seen a collection of wizards, then this random boy that they know Harry lives with and doesn't like? That would make him stand out even more. The same logic applies if they were to disguise him as a random muggle from the village, or wizard from the wizarding community (not to mention they need some "part" of whoever they want to change in to).

On top of this, even if this ruse was successful, if the real Dudley was seen in future he would likely be killed instantly by the Death Eaters as they would suspect it was Harry. In other words, it would draw unwanted attention to Harry's family, putting their lives at danger. Again, this logic applies to any other wizard/muggle they could disguise him as.

They could have made Harry change into another member of the party, e.g. Hermione or Ron, but this solution isn't really any different to what they actually did in the end. Doing this would have resulted in two or more Hermoiones/Rons and would have left the same problem for the Death Eaters that their actual solution (the Seven Potters) did.

Therefore, their final solution of disguising everyone as Harry seemed to be the safest solution - albeit, still very risky (as the death of Moody showed).

Now, turning to the books as evidence - firstly, from Snape's pensieve flashbacks:

“You will have to give Voldemort the correct date of Harry’s departure from his aunt and uncle’s,” said Dumbledore. “Not to do so will raise suspicion, when Voldemort believes you so well informed. However, you must plant the idea of decoys; that, I think, ought to ensure Harry’s safety. Try Confunding Mundungus Fletcher. And Severus, if you are forced to take part in the chase, be sure to act your part convincingly. . . . I am counting upon you to remain in Lord Voldemort’s good books as long as possible, or Hogwarts will be left to the mercy of the Carrows. . . .”

Now Snape was head to head with Mundungus in an unfamiliar tavern, Mundungus’s face looking curiously blank, Snape frowning in concentration.

“You will suggest to the Order of the Phoenix,” Snape murmured, “that they use decoys. Polyjuice Potion. Identical Potters. It is the only thing that might work. You will forget that I have suggested this. You will present it as your own idea. You understand?”

“I understand,” murmured Mundungus, his eyes unfocused. . . .

This shows that it was Dumbledore and Snape's idea - so Dumbledore himself really had thought this gave Harry the best chance of escape. Ths strategy essentially served two purposes - to give Harry the best chance of survival and to firmly establish Snape's reputation as Voldemort's most trusted ally.

The books also describe why the solution of apparating couldn't work. As explained in the seventh book:

Moody dropped his sacks at his feet and turned to Harry. "As Daedalus probably told you, we had to abandon Plan A. Pius Thicknesse has gone over, which gives us a big problem. He's made it an imprisonable offense to connect this house to the Floo Network, place a Portkey here, or Apparate in or out. All done in the name of your protection, to prevent You-Know-Who getting in at you. Absolutely pointless, seeing at your mother's charm does that already. What he's really done is to stop you getting out of here safely.

"Second problem: You're underage, which means you've still got the Trace on you."

"I don't -"

"The Trace, the Trace!" said Mad-Eye impatiently. "The charm that detects magical activity around under-seventeens, the way the Ministry finds out about underage magic! If you, or anyone around you, casts a spell to get you out of here, Thicknesse is going to know about it, and so will the Death Eaters". "We can't wait for the Trace to break, because the moment you turn seventeen you'll lose all the protection your mother gave you...We're going to use the only means of transport left to us, the only ones the Trace can't detect, because we don't need to cast spells to use them: brooms, thestrals and Hagrid's motorbike."

So any attempt to transport Harry would have resulted in his whereabouts being immediately known - and as we see a few lines later on, the group really believed they had misled Voldemort and thought they could safely get Harry out:

"The one thing we've got on our side is that You-Know-Who doesn't know we're moving you tonight. We've leaked a fake trail to the Ministry: They think you're not leaving until the thirtieth."

So ultimately, they couldn't apparate without drawing attention to themselves. Harry then turns seventeen at the Burrow. This conversation takes place at the Burrow:

Hermione: "What if the Death Eaters have found a way to put [the Trace] on a seventeen-year-old?"

Ron: "But Harry hasn't been near a Death Eater in the last twenty-four hours. Who's supposed to have put a Trace back on him?"

So Ron, from a wizarding background, seems to suggest the Trace needs to be manually reapplied. Therefore, after his birthday, they could apparate safely, but not before.

They had few plans.

Plan A:

Moody dropped his sacks at his feet and turned to Harry, “As Dedalus probably told you, we had to abandon Plan A. Pius Thicknesse has gone over, which gives us a big problem. He’s made it an imprisonable offense to connect this house to the Floo Network, place a Portkey here, or Apparate in or out. All done int he name of your protection to prevent You-Know-Who getting in at you. Absolutely pointless, seeing as your mother’s charm does that already. What he’s really done is to stop you from getting out of here safely.

Harry Potter and the Deathly Hallows - chapter 4, The Seven Potters

Plan B:

“Second problem. You’re underage, which means you’ve still got the Trace on you.”

“I don’t—”

“The Trace, the Trace!” said Mad-Eye impatiently. “The charm that detects magical activity around under-seventeens, the way the Ministry finds out out about underage magic! If you, or anyone around you, casts a spell to get you out of here, Thicknesse is going to know about it, and so will the Death Eaters.

“We can’t wait for the Trace to break, because the moment you turn seventeen you’ll lose all the protection your mother gave you. In short: Pius Thicknesse thinks he’s got you cornered good and proper.”

Harry Potter and the Deathly Hallows - chapter 4, The Seven Potters

The reason for 7 potters

“We’re going to use the only means of transport left to us, the only ones the Trace can’t detect, because we don’t need to cast spells to use them: brooms, thestrals, and Hagrid’s motorbike.”

Harry Potter and the Deathly Hallows - chapter 4, The Seven Potters

Why use Harry's face. It was a honeytrap. There was two specific reasons for that.

“We think the Death Eaters will expect you to be on a broom,” said Moody, who seemed to guess how Harry was feeling. “Snape’s had plenty of time to tell them everything about you he’s never mentioned before, so if we do run into any Death Eaters, we’re betting they’ll choose one of the Potters who look at home on a broomstick. All right then,”

Harry Potter and the Deathly Hallows - chapter 4, The Seven Potters

Harry travelled on Sirius's bike. So that he won't be targeted first. Secondly he travelled with Hagrid, the least qualified of the wizard among the 14. The plan was to distract the death eaters, so that they attacked fake Harry's travelling with Moody, Kingsley, Lupin, Tonks, Arthur and Bill before attacking real Harry travelling with Hagrid.

This was done to distract any death eaters waiting for them outside and provide enough time for real Harry to escape.

Voldemort and company knew what day Harry was leaving and would be watching.

So, why not use random muggles? Even if they did, Voldemort and company would still know that Harry was one of the group leaving the house that day. If they left in a large group as planned, the confusion caused by the Polyjuice potion is practically the same if there are 7 Harrys as if there are a bunch of random muggles. (Technically, the 7 Harrys are slightly less confusing.)

Now, let's look at a few other points:

1. If the people of said group left in smaller groups, all random muggles, it would take less manpower to kill them and the Death Eaters would just overpower each tiny group regardless of the random muggle faces. (This would also occur if there were 7 Harrys. Each of the 7 would certainly die.)
2. If they used all muggles, they are going off of pure chance. The answer that was posted by Vishvesh proves that part of the plan was to use psychological warfare. By using 7 Harrys and putting the real Harry in the sidecar of a flying bike instead of on a broom and by putting him with Hagrid and his poor wizarding abilities instead of with Kingsley or Lupin, they increased the odds significantly that he would be attacked last and focused on least.
3. Harry's hair was readily available for the Polyjuice potion. Preparing a Polyjuice potion of 7, or 14, or more, muggles would be a more tedious task.

It was probably more confusing to the enemy and way more convenient to the good guys to just make 7 Harrys.

As explained here: https://scifi.stackexchange.com/questions/29428/in-the-battle-of-the-seven-potters-why-didnt-harry-take-the-polyjuice-potion

Being Harry was in-fact one of the safest people to be in that situation; given that there were extremely strict orders to capture Harry and not kill him!

Having more than one Harry was only a ploy to confuse the Death Eaters, so that perchance they would not be able to capture the real Harry.

There is one main reason for the need of "7 Harrys". Harry was the only one in the wizarding world who could end Voldemort's life and reign of evil. Therefore, it was imperative that he remained alive. Mad-Eye had the idea that the other members of the Order involved in moving Harry from Privet Drive should drink Polyjuice Potion to appear as Harry because it meant that when Voldemort and the Death Eaters attacked (which they were definitely going to do because they knew Harry was being moved that night) they would not know which was the real Harry, making it harder for Voldemort to kill him. Basically, everyone who took that potion was sacrificing themselves in order to keep Harry alive for the sake of the wizarding world.

If a group of known wizards transfer a person-who-is-not-Harry it is obvious who to target.

If a group of known wizards transfer someone - only that someone is not present - but there is an extra of one of the wizards, it's obvious which two to target.

If a group of known wizards transfer someone disguised as a known wizard who remained at home, information on who is at home selects the target, someone acting out of character selects the target.

You get Harry act out of character by making him uncofortable, you got seven wizards mostly identical.

Harry taking polyjuice potion wouldn't work.

1st case: if he changed into an unknown person, the deatheaters see a bunch of people entering the protected area. Then they see those same people leave + 1 unknown. Well, obviously that's Harry no matter how he looks. Might as well just cover his face with a scarf and it wouldn't be much different.

2nd case if he changed to one of his protectors, he would just put a target on himself and the protector he changed to. For example the deatheaters see 2 Moodies, so obviously 1 of them is Harry. So they kill/capture both just in case.

In the 2nd case, If they left the spare protector in the house, they just condemn him to die since the deatheaters are bound to search it.