Why do the kicks have to occur simultaneously?

I love the question. Here's a summary from a very tight explanation (with some drawings):

Inception and Kicks

There were two types of dreams. The simple construct with a mild sedative and the more complex construct with a strong sedative.

In the simple construct, one kick is enough. Kick can be either to the self (eg: Arthur shot in the head) or a kick from above (eg: Cobb dunked in bath tub). The sedative is light so there is no synchronization required. In real life this can be equated to sleeping and dreaming about falling, the fall in the dream wakes us up. Alternately someone wakes you up by pushing you off the bed.

In a strong sedative, complex construct dream - a single kick is not going to be enough. A person needs two synchronized at the same instant. One kick to the self and one from the level above. The synchronization is done using music. In the real world, this can be equated to being under total anesthesia. Neither would a bad dream fall wake you up nor would pushing you off the operating table. The reel part here is that if some how pushing you off the table can be coincided with the fall in your dream, you can wake up.

Now, in the act of inception on Fischer's mind, they are in a complex heavily sedated dream. The only viable kick here is falling because death will take you to limbo. As mentioned in another answer, waiting it out is not going to work because weeks and months would need to pass in the lower layers. Waiting it out is a possible option only in the first layer because there is limited time dilation. Also a single kick from a level above or jumping off a cliff in the current level wont work as a kick. The dreamer is far too sedated for that. A person needs to fall in the current level and the level above simultaneously so so they plan. The following kicks (falls) are planned:

Level 1 : Van Falling
Level 2 : Falling by one floor (blowing up the floor)
Level 3 (inception level) : Falling from the building (blowing up the structure)

Of course, things don't go as plan. Hence they improvise on the kicks. Here is the set of synchronized kicks that each one receives per level of dream. Point to note, no two people receive the same two kick pairs.

Here's the image from the link:
_{(source: thisisbarry.com)}

So, other than dying in limbo, everything else needs a pair of synchronized kicks. For more details, read the link.

There are three ways to wake up from the dream: wait for the timer to run out; die; or receive a kick from the next level up. Because of the sedatives, dying wasn't an option; they'd just end up in limbo. And because of the multiple dream levels, waiting for the timer to run out wasn't an option, because they'd all be killed by the projections in the time involved. Kicks were the only way out.

Now, it was clear from the movie that there was a definite problem with kicks coming too soon. I don't think the exact consequences were made clear, but we can deduce some things. If it's a two-level dream, in the first level is one awake guy (the first-level dreamer) and some number of sleeping people who are in the second level dream. If a kick comes in the first level, the first-level dreamer is definitely going to wake up. We can assume that, at least under the sedated dream we saw in the movie, the remaining dreamers don't also wake. Otherwise there wouldn't be a problem; the top-level kick would have woken them all the way down the chain! So for the kick to wake you from the first level, you have to be awake in the first level. This may be a consequence of the sedative, and not a general rule of dreaming. Or it may be a function of the differing time scales; if time is moving subjectively faster on the lower levels, what would seem like a sudden acceleration one level up might subjectively be too slow for you to notice.

What happens if the first-level kick comes while you're still in the second level? The guy dreaming the first level just woke up, so if you wake up from the second level after that, what are you waking up to? Limbo seems the most likely consequence.

So the kicks needed to be as late as possible, to have as much time as possible to complete the mission. But they still needed to come in a definite order, to avoid limbo. Thus the tight synchronization required. But they are definitely not simultaneous, as the question asks. We see characters waking up from level after level, in sequence.

I found the following dialog that I think answers my question:

Ariadne: So, do you use a timer?

Arthur: No, I have to judge it for myself. While you're all asleep in five-two-eight, I wait for Yusuf's kick.

Ariadne: Well, how will you know?

Arthur: Cause music warns me and then when the van hits the barrier of the bridge that should be unmistakable, so we get a nice synchronized kick. If it's too soon we won't get pulled out, if it's to late I won't be able to drop us.

Ariadne: Well, why not?

Arthur: Because the van will be in free fall. Can't drop you without gravity.

I believe the "we won't be pulled out", means that the kick from the level above won't pull them out before the fall kills them and they end up in limbo. I've also heard another theory that the kicks need to be synchronised because this is the only way to wake them up with the strong sedative, but I don't believe that this is well supported.

It's probably to avoid limbo. One enters limbo if they die in a dream while being heavily sedated in reality. One other concept to grasp here is that when a higher level dreamer cuts out the dream, the lower level starts collapsing/the other dreamers in lower level might get killed by projections. The second part of this concept is however, my own one.

Let's say we are in second level. If one among the dreamer dies while dreaming, he/she gets the kick to the higher level (and goes to first level). But the people who are still dreaming will experience (in second level) things such as the dream collapsing, and also may be the projections (of the dreamer who exited) trying to kill them.

When Arthur was killed while Cobb was trying to extract Saito, the dream started collapsing, because Arthur was its architect and he woke up in the higher level. Now, Arthur didn't have his subconscious projections in the dream space in that level, because afterall he is just an architect. The subconscious was Cobb's and Saito's.

This leads to a chain of questions which answer themselves.

Now, if Arthur wasn't the Architect of that dream, or instead Cobb was killed in that level - what would have happened to Mal and Arthur in this dream? Would Mal have tried to kill Arthur because he was the dreamer? Could she/ other projections killed Saito and Arthur instead? Would Mal exist in such a scenario? What would happen to Cobb's subconscious that existed till then in that level? Did the dream start collapsing only because Arthur was the architect and does that mean what you've created only will get destroyed? One thing we should know is that when you see other persons subconscious in your dream, it automatically becomes your subconscious too, but you know that it doesn't belong to you. Does that mean the person's projections still exist and due to the heavy disturbances created by you, the dreamer, the projections will start to kill you too?

Taking into account the possible answer to those questions, let's assume the worst case scenario that even the projections (of the person who exited) would also try to kill the dreamers (who are still dreaming) in this level. However, this wouldn't change my answer.

In the movie, what happened to those people who didn't get the kick as planned? Saito and Cobb - they entered limbo.

And because the dream will collapse and may be the projections might kill you at a lower level, you have to get out of it as soon as possible. So, all of them got their kicks at the same time to ensure no one dies, and to ensure no one enters limbo. At the first level (van ride one), they probably would have kicked Fischer in such a way that he would wake up a little later after they wake up - so that no one doubts anything has happened. We have to assume that they didn't let the security kill them in the first level and they kicked themselves to wake up in the aeroplane, assuming you want to take the ending for reality.

I think that much explanation settles it. Hope that helps!

I think the idea is that they need tranquilizers on drip to be able to maintain such a complex hierarchy of dreams, and while this is pumping into them, you need two kicks to wake you from a dream. One in the dream, one on the layer above.

For example: in the hotel. Arthur kicks the rest of the group (and himself) using the elevator, and simultaneously the van hits the water. In the snowy mountains, Ermes kicks everyone by dropping the whole building, and simultaneously the elevator kick happens. Given how dangerous it is to hang around in the lower layers of dreams, it makes sense to kill two birds with one stone, and use the 'wake people from the lower level' kick as the 'wake us up in the layer above' kick by synchronizing them.

This simple plan is complicated by them missing the first drop from above (the van going off the bridge) meaning gravity was lost in the hotel.

It's also important to note that the dreamers have no way to communicate with the layer above, and no way to impart the kick in the the above dream. so once they've agreed to this schedule, they have to follow it. As far as they know, the dreamer of the above dream will wake up on schedule, and of they aren't awake (/waking up) in that dream when the time comes to wake up in the above dream, they'll fall into limbo - hence all the rushing around to stick to the schedule.

If the dreamer and architect of a level wake up/die, one of two things happens:

those left behind/alive either

1) go into limbo (since they die [?] as a result of the level crumblind down [?]), or

2) wake up.

If they go into limbo, they can just kill themselves and wake up, right? So why take the trouble to synchronize jumps, if all they had to do was shoot themselves? It wouldn't make for an interesting movie, true, but it still is a viable solution, isn't it?