If you're a good NFL player, but not a stud, you're doing well to end up with an 8 or 9 year career.
Sure they make good money for a short period of time, but they also have a load of health issues that come from banging heads for years and years into their lives, and that money has to last them for the rest of their lives.
They aren't all making the Tom Brady/Peyton Manning money either.
You have to keep in mind that you have to think about it from the perspective of the average NFL player. They all make good money, but for most of them, it's a big deal when you start adding two more games to their schedule and two more games of pounding for them to take.