Is it anything to do with High School/College games being played so differently to the NFL?
Or is it until they are part of a proper program and behind a good O-line it's difficult to compare?
Over 100 FBS colleges, over 100 FCS colleges, all allowed to have 105 'active' players on the roster.
So that puts the number at around 20,000+ players on the college rosters, right.
The QB in a top team (Alabama, Clemson, etc) is let's say competing (as his teammate is on the opposite of the ball - OL v DL), against some teams who have players who could be the 10,000th, the 15,000, the 18,000 or 20,000th best player in college football, if you ranked them all on their abilities.
So that QB who had it so easy against another college team gets drafted to the NFL who have 1,500-1,600 registered players in the whole NFL.
He's not playing no more against anyone worse than at least the top 2,000 players in the game, give or take the 300-400 at the bottom end of the rosters going on and off the roster during training camps or injuries during the season.
So it's at least 10 times harder to be a success in the NFL than college football, particularly QB's which is the toughest position to play.