It's one thing to run around and pull plays out of your ass to cover for your lack of sound fundamentals in college (right, Johnny?), it's another to try and do it in a league where any player and half the coaches on the opposite sideline can easily break you in half for trying it. Mobile/scramble QBs that made it work in the NFL also had the tools to stay in the pocket and deliver and the knees/body composition to take a hit. RGIII had neither.