I can't speak for anybody else, but if I go down when I'm riding more than about 8-10 mph, it's bone-jarring, and almost knocks the wind out of me. I'm also left drained; weak and shaky; and very light-headed. It takes a long time to wear off. How pro racers manage to get right back on their bikes after a crash and keep on racing is beyond me. They must be exceptionally tough, both physically and mentally.
Why a crash makes me feel that way puzzled me. I ran it by none other than Joe Friel, author of the Training Bible series. As I remember, he said something about a crash causing a flood of stress hormones to be released into the system.
Maybe the reason pros can pick themselves up off the ground and go on racing is that it's happened so often that they've become "immune" to it? If so, they certainly paid a high price for the "vaccine".