Remember that many athletes play sports because they are good at it. Not because they love it. They are good and earn a lot of money with it, but their passions lie elsewhere. That can sometimes lead to earlier retirements. It's the guys who really love it who prepare super hard every offseason, etc.
You see it a decent amount of time in different sports. If Burnett wants to do other things with his life, he still has made plenty, has an excellent pension, and plenty of deferred salary to keep his family comfortable for many, many years.