The problem here is that you more than likely never been an athlete in your life. And this is not a insult to you so don't take that way. Therefore, when you, like most people, see these athletes leave to pursue a career in the professional ranks. Nine out of ten times you think that it is only money driven. Although money is a major benefit, however there are other major factors at play. So, depending on the sport and the position being played the clock starts much earlier than your average student that goes to the same university, graduates and applies for a job. For a RB in this case, he has been training for an opportunity like this since he was able to carry a football. When he finally got a chance to compete at the highest college level his job interviews started at the first practice. From that point on he was at risk of losing it all to injury and playing time. So in order to understand what is truly at stake here you have to look at this from a career in sports point of view and not your average 9 to 5 job point of view.
Let's deal with returning to school to complete his degree because of free room and board etc which he more than likely live off campus anyway but for kicks lets do it. Fact he is a Jr, so he will be completing his Jr year of college. At the most he will have about 20 credit hrs left. From his point of view, why risk injury returning when he will have to run the football through spring training, summer camp, fall camp and no guarantee of playing time? So when you put this in prospective, it's like being up $5 million at the craps table and someone is like go all in and roll again. He would have to be crazy as ****!!