It just happens it every sport, mate. Look at Cesar Chavez. Do you know how many taxi drivers and banana salesman he had to beat to get that record? And with boxing, the way the contracts work is just very different. What boxing does thats great and bad, is they out these young guys on salaries. So not only do they get paid by fight( big or small) the promising ones get a monthly wage as well. And what happens is the promotion is putting money into these young kids, and want to protect their investment until they're ready. Then fighters get a say so in the contract with things, and that happens when the promotion is making shit ton of money from many different avenues.
There is good and bad for it all. The bad is for the fans because we have to sit and watch the politics, but it's also good for the fighters in the short and long term financially wise