Smart rich people never get tired of opportunities to make money.
They also like getting shit for free.
Cameo means nothing. The amount of celebrities on cameo who have a huge net worth, suggests exactly what I said. If you give rich people a way to get richer that's easy, many of them will take it. Some celebs are making 250k a month just recording cameos. An in-demand celebrity can make millions per year on Cameo, which is why so many are doing it. Easy money just to tell some civilian happy birthday or something.
Rizin is easy money for little work for Mayweather. He may be worth 400 million (or whatever it is), but 9 million (or whatever they are paying him) him is still a shitload of money for a few minutes work with no real risk.