most do stay healthy for 1-3 performances a year.
but it's the nature of the sport. preparing for a 3 round fight at the highest level takes an unbelievable effort, never mind a 5 round fight. we all scoff at people who gas in fights, but very few people have actually gone through the training regimen that's required if you want to fight hard for 15 minutes.
if you hold back, you're not as sharp or as well conditioned as you should be. if you don't, you risk getting injured. it's fighting. it takes one tiny fuck up and you get hurt. imagine training 10 times a week for 8 to 9 weeks and not fucking up, ever. it's a miracle people don't pull out of every second fight.