This isn't a traditional career though. Most college guys don't make it to the NFL. This is quite possibly the short window in their life where they have a highest earning potential.
If you're a AAAA college player (really good in college but probably UDFA in the NFL, if that), this is the point in your life with your highest earning potential. In most careers (not in football), your highest earning potential is in the mid to late part of your career, so it makes a lot more sense to 'pay your dues' so to speak.
And all of that is without acknowledging the injury risk. Remember Marcus Lattimore with his significant knee injury in his final college game? Under NIL, he would have banked some pretty decent cash before that injury.
It's a very very (VERY) flawed system, but I do not blame players for doing what it takes to maximize their earnings during what is a very short window for most of them.