Not to mention a large amount of NFL players don't play for very long. A quick Google shows an average player career of just 3.3 years, which is ridiculously short even by modern sports standards. The average goes down depending on which position there is as well. Most other major sports have an average of 5 - 8 years for careers.
Imagine getting to the NFL and then only staying for a couple years. Sure you're set for life with that paycheck but expecting somebody who's like 24 years old at most to be able to be financially responsible with that money is just absurd.