Usually because later 1st round picks are "reaches" from teams desperate for a QB (e.g. Ponder, Tebow, ...)
They are not much smarter - I think most of the people on this board would be much better drafters than most of the GM's.
The problem with most GM's and coaches is that either a) they are stupid or b) they think they can outsmart everybody and end up being stupid or c) are desperate to make the big splash.
If you just stick to basic common sense you will do OK