In Inception, why do the kicks on different levels have to all occur simultaneously, rather than one after another?
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.
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.
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 found the following dialog that I think answers my question:
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.
In the movie the plan is to enter Robert Fischer's mind while he is flying. So they only have a limited time. One member stays behind to "kick" they to the next level. They have to go deep so they can plant a idea that seems like it comes from Robert Fischer's own mind which is planted idea: that his father wishes him to be his own man and that he must split up the conglomerate.
Cobb has done this before (which one finds out near the end who and what he did). The reason for the multi-level kick is so they can rapid ascend the levels and in theory wake up and hide everything before Robert Fischer wakes up.