Professional prizefighting is mostly about fighting for money, but that's not the whole story: the competitive nature certainly plays a part as dos the drive for personal fame, to be somebody.
Some of these guys really want to prove they're the best, or can at least compete with the best.

Take mixed martial arts: most ufc guys, even the champions, are fighting for peanuts compared to the lion's share of the ppv dollars that goes to dana white and the fertittas. For them, apparently, the money isn't the most important factor.

When a guy's only in it for the money, that's when fight's end up not being on the level, when guy's throw fights and take dives if the money's right, and some prizefighters with real Heart just don't have it in them to lay down for anybody at any price...