If you are undefeated and most of your marketability is based on that, then you're allowed to duck, well not so much duck, as pick out the opponent that best suits the risk/reward element that comes into play. You start off good, look promising, maybe even win a few titles. But when you get to making 8 figure paydays each time out, you have to be careful because one loss could mean the end of those 8 figure paydays.
Same goes for team sports. You'll see a guy tear it up for the first several years of his career, and then get the big multi-year, 100 million plus contract, and then some of those guys play to not get injured and get paid.
It's how sports is, at the end of the day, it is a job, a business. There's money to be made, not just for the athlete, but promoters, networks, cornermen, officials, sanctioning bodies. So you do what you gotta do to keep the money flowing and keep the illiusion up that you're in it for the sole purpose of winning.
The prospects of millions and millions of dollars, extremely dillutes ones passion to excel at something.