Fans of bodybuilding can be strange; after a bodybuilder retires he/she is going to lose muscle mass and probably gain some fat. Why would they try to retain it all what would be the use, to walk on the beach as the biggest guy?
If the average pro has been training for 15-20 years at retirement (8-10 years to become pro and another 5-10 years of competing at the pro level) then he will probably want to take a break from the extremely restrictive eating schedule, the intense weight training (many actually don't seem to train any harder than the average gym rat), the extensive drug protocol, the constant cardio, and the politics and BS of the industry.
In basketball, football, tennis, soccer, rugby, baseball, sprinting, what ever the sport an athlete will not retain his/her ability or contest level when they retire. Heck most athletes start decline well before they retire.