TL;DR: Athletes do make a ridiculous amount of money; but if they weren't paid so much, the owners would just make even more money than they already do and make the distribution of wealth even more unequal.
They make so much money because they attract a lot of fans, and the fans bring in money
The league has to pay the players big time to keep players interested in playing
It's a business
Viva la Rhinoceros Party