That's because the qb position in college ha become so bad that quality passers aren't coming out on a regular basis. Think about it. guy comes in gets red shirted for his 1st year plays 2 seasons then goes on to the nfl. He now can't play from behind center or read a defense. He is a in the shotgun 1 read player. They can get by for a couple of seasons and if they can't transition to be an actual qb they fade away like Kap and RG3. That's why most teams are so inconsistent.
The last of the true qb's are Eli and big Ben and they are in their mid thirties