They self test for peds and pull out because they don't want to get caught. Back in Pride FC not many fighters pulled out, but in UFC its very common.
i don't know much about mma but one of my friends loves it and he says a lot get injured because hard sparring is the culture in a lot of gyms especially in brazil right up to fight date
Back in the "old days", not many people pulled out in Pride or UFC because they were fighters. They didn't do it for the money, because there really wasn't any. But, injuries are common in many training camps/gyms because they dont' know how to train. They have this macho thinking that "alive training" means that you have to go hard and all out all the time. It's just now that gyms are realizing that you can work aspects of your game with drills and various levels of resistance and still improve and more importantly stay healthy.
I think def so in Brazil. I saw Erick Silva's training vids for one of his fights a while back, and his stand up and ground game sparrings were brutal, abusive even. I couldn't believe it, At one point he was considered the next Anderson Silva, but his career in the ufc definitely reflects what I saw in that video.