Why is Anderson Silva still fighting
You the Greatest Of All Time, you don't have to do this
Because the UFC doesn't pay their fighters worth a damn.
It's slowly starting to get better for the fighters (thanks to the likes of McGregor and Diaz holding out for more money, media attention, etc.), but the UFC has gotten away with paying their fighters in peanuts since day one.
Ain't no one expecting them to get paid stupid money, but when you have a card that draws 800,000 PPV buys and generates over 60M in revenue the main event fighters should be getting paid more than $100,000-$500,000.