Because it's not the falling, it's the jolt of landing. When in free fall, you are not actually accelerating. In terms of landing, though, it doesn't matter if you hit the ground or if, say, the bottom of an elevator hits you.
I haven't seen the film since it came out, so I may be mistaken. Didn't Cobb demonstrate to Ariadne that you wake up when you sense yourself falling? By tipping Arthur's chair over as he was dreaming? Arthur (if it was that character) didn't fall on the floor, there was no jolt. Also, doesn't the van land in the water, and no one wakes up?
According to the Inception Wiki, we're both right.
> One method used to awaken from a dream within a dream is called a "kick", which is the sensation of falling, hitting water, or a sharp jolt that can startle the sleeper awake.
Although I still think it's terribly inconsistent, if the details matter. There's too many different kicks with too many subtle differences and similarity for me to go into in a reasonable-length post.
OK downvoters, you're right. In the movie, they explain that a kick is the feeling of falling. So why didn't Cobb wake up when he was falling towards the water, but after he fell in? My guess is that the water invaded the dream and he drowned, and exited the dream via death. But they called that a kick, too. As we've seen, the movie is hardly straightforward. And if one of you had bothered to comment "they say in the movie that it's the feeling of falling" then I wouldn't have had to rewatch half the movie.
