Why do these guys doesn't retire at their peak.
There are a variety of reasons, but if you have a contract with a company which says you need to compete at least once or twice a year, then you need to compete. Hopefully, you will come into shape when you are up there.
As you get older, your body changes and what may have worked in the past doesn't necessarily work. You need to change your diet, training, etc. And of course, those last crucial few days, where it can make or break a show. You can look great two days before a contest, and come up smooth at prejuding.