Good question. Some guys do fight 5-6 times a year though.
Im not a fighter, but i am an endurance athlete. I know from my personal experience that it's not the event, but the training that takes it out of you. Niggling injuries build up, aches and pains take time to go away, and it often takes me weeks and weeks after an A race before i feel strong again. Fatigue hangs around much longer than i think some people realise.
That said though, i race 5 or 6 times a season (2008 will be 2x half ironmans, 1x full ironman, 1x olympic distance, 1x sprint) and thats between May-September. I would think fighters could fight more too, even with proper rest and tapering.
Demian Maia may be the Fred Astaire of jiu-jitsu, but it’s too bad Silva is the Gene Kelly of kicking everyone’s ass.