I had two close friends/associates, one made it to the final cut in Oakland 1975 and the other had a successful college stint at Nebraska in the late 1990's. They explained that many players, especially QBs have the intelligence, physical attributes and the arm for success in the NFL; the just can't process the gane at NFL speed. Once you leave high school to a major college the speed of the game and players including size and strength is far cry from the varsity, the college world a different planet and the NFL another universe.