Footballers don't head the ball with anything like the force of a left hook off a 12oz glove. It's more a build up of frequent small hits that can potentially cause long term brain damage. That's something you can tackle by frequent monitoring, brain scans, memory tests, stuff like that, which would then lead you to take action at your leisure when the results point to a problem. It's very different to getting hit so hard in the head that you die there and then, without the choice or opportunity to do anything about it.
To stop people dying you either use head gear or use smaller gloves. Or change the rules to have shorter rounds, or the ref calling it off after the first knockdown. The worst culprit is definitely the gloves. The opponent has their hands wrapped up like sledge hammers, and the gloves to protect their knuckles, it gives them a free licence to put the weight of the earth into each punch, which is insane and unnecessary to knock someone out. Reduce the glove sizes, and you'll have to time the punches and catch them with more accuracy on the chin, with short sharp shots. No more of these brutal 12 round beatings to their heads.
Deaths in the boxing ring and deaths on the football pitch are as rare as rocking horse shit thankfully.