system
A chat on subreddit /r/NoStupidQuestions
human
Volume. It’s not just the NFL kids as young as 10 play football. MMA and boxing don’t have college teams. Team size.  52 man roster v 1v1

Rules. In MMA/Boxing there are weight classes so people are paired based on their size. In the NFL a QB can get hit by a 300 lbs EDGE. There’s also other ways to win in MMA submissions. 

Acceptance. People accept that combat sports are dangerous and can result in head injuries. There’s a lot of emphasis on refs stepping in to protect fighters. In the NFL players play through a head injury

Edit-time off. An NFL season is 18 weeks for the regular season. Players play week in week out. Boxing/ MMA is event based so more time to heal
gpt
Boxing does have a few college teams, but nowhere near as many, and with a much higher degree of padding than professional.