Brady's a bit of a prodigy poise wise, but he still sat a year and built his arm and physique up.
It's simply no accident how many QBs who are thrown right in flame out and how many "lesser" QBs that get a few years look like the top prospects. Cassell, Hoyer, Fitzpatrick etc.
I'd love someone to do a study. There is so much physical and mental preparation to getting up to game decision speed that putting that pressure on 90% of draftees will cause discouragement and bad habits. There's always the 10% naturals, or very experienced big program guys, of course.