This still amazes me. I've seem all of these reports of all these "QB gurus" that are former NFL and college QBs holding these camps and the NFL is always struggling to have enough solid starting QBs. Todd Marijuanavich's dad started coaching him as a toddler and he turned into a good college QB who was drafted by the NFL but didn't pan out, mainly because he got into drugs after dealing with his dad but the really good QBs are few and far between.
I'd have to say, as compared to tough positions in other sports (pitcher and catcher in baseball, goalie in hockey, point guard in basketball) having 300# guys trying to kill you has a big impact (David Carr, Jim Everett, etc.). Reading defenses, making the right and accurate throw while thinking about the defense coming after you seems to be a difficult task indeed.