The IRS audits plenty of people with low incomes. Roughly 2% of people that declare $0 income get audited, which is actually higher than any other income group below those who make millions of dollars a year.
I'm not sure what you mean by "Squeezing people with zero income who pocketed 75k".
I can't speak specifically to college football athletes, but that is almost an entire year salary for me.
If I took home an entire year's worth of salary under the table and the IRS found out, I would be absolutely ****ed. I can guarantee from the stats on the matter and people who have been audited for far less, they would not write me off as a "minnow" that's for sure.
Of course many, many, MANY more get audited who make $75k or less...... or are in the 0 bracket....... that's, sadly, a lot of America. If you make a bunch of money, you pay someone who makes you appear NOT to make a bunch of money.
But when it comes to chasing down college athletes who got paid, it's just not worth it. The kid denies it, the program isn't going to name names or risk never being able to recruit again.
Reggie Bush got sued by a booster in civil court and settled. That's a court record, something the IRS can use. Heresay, "Coach Pruitt and Coach Nieds were giving out thousands......." with no record, that's not going anywhere except for the NCAA which ISN'T a court of law. Plead the 5th and the IRS is stuck trying to chase real evidence.
It's not like the university was cutting checks to players or Pruitt was handing them personal checks. The credit card stuff is

dumb but unless Nieds was rolling with a Black American Express, how much money can you spend at a strip club...... wait...... don't answer that.
This is the darkest of dark money by default. That it landed on the Chancellor's desk and became a "we can't cover this up" situation is sad but relatively speaking, this isn't a ton of money.