system
A chat on subreddit /r/NoStupidQuestions
human
In the 1950's through the 1980's, Boxing was one of the biggest sports in the United States, but has been in steady decline for the last 50 or so years because of a lot of the safety concerns that you list here.

I think the big concern people have with the NFL is that football consists of very large teams and recruiting happens in high school or even younger, so the safety of children is constantly being called into question. In the United States, schools very rarely (if ever?) have sponsored Boxing or UFC matches, but it's almost assumed that they have a football team.
gpt
Youth boxing features padded headgear and strict rules about duration of a fight and how many knockdowns can happen.   


Youth MMA largely doesn't exist. You pretty much have to be a minimum of 16, and in most places 18, to participate in the full-contact version. Kids who want to do MMA for real generally train the different disciplines, but only really participate in the BJJ tourneys, where head injuries are rare, before they're 18.   


Meanwhile, you can play your first season of real, tackle football starting around age 10-11.   


Football is horrible for kids, particularly as they get much faster, heavier, and stronger in their teenage years and can deliver much more kinetic energy and whiplash.