Because whilst the sport might look violent, isn't actually about being violent. The same way ultramarathon competitors run through immense pain but running isn't about self-harm.

When you do martial arts you don't enter the ring thinking about how you're going to punch your opponent's teeth in, you think how you're gonna hit them without getting hit yourself. Even in sports that allow you to take hits (I did Longsword fencing, where we wear protective gear but count every hit that would end the fight between two naked fighters) and therefore hitting hard is advised, you're in a "let's see who is stronger" mindset, not in a "I'm gonna fucking kill you" mindset.