Let's make a difference between the bastards you are talking about Johnny and real football fans. The scum you refer to do indeed have a "sport". It is fighting. They don't like doing it in gymnasiums and clubs where they have to fight people of near their own weight and ability. I could offer several suggestions as to why that is not the case.
The majority of football fans are no more violent than the average knitter of woolly mittens. Unfortunately, the scum attach themselves to football crowds and thrive on the anonymity as a blanket for their thuggery. I have attended football matches for over thirty years and they have always disgusted me.
I do not have either the inclination or physique for violence. (At five foot four, I am half the size of nothing). I am a pretty average football fan. I did all my fighting sports in gymanasiums (boxing and judo as a kid and now karate). I have never fought anyone in any other place. That makes me about as violent as the average football fan. The scum does exist, but I don't deserve to be judged by their actions.
In short, they exist, but they are a pitiful minority.