Why? Why do MMA fighters do this?

Money.

Former UFC middleweight champion Anderson Silva said over the weekend during the chaos of his Julio Chavez Jr boxing match that he has two opponents in mind for his next go. The Paul brothers. It is a possibility now that he holds a split decision victory over Chavez Jr. Please no.

The 46 year old spoke with TMZ Sports about it:

His age makes him a perfect candidate for a freak show boxing match with Jake or Logan Paul. They tend to choose well known MMA fighters past their prime or that have never, or no longer real danger in the striking power department to call out. Then again, Silva still has some power left so maybe they won’t? Please no.