violence and sports

I usually wonder why is violence the answer of everything?. Why in team sports such as football, basketball or baseball, whose aim is to join people, it ends separating us?
We become rivals against each other and I still wonder why people behave like that, why does the man, the most intelligent one, and in some countries like England boasting to be the most politest one, behave like a wild animal, fighting to keep his "reputation".
Those firms only care about going to their pubs, get drunk and having a great fight against the other team supporters. And in spite of the effort of the police, the problem is still there.
Personally, why don´t the football players, "their own idols" campaing against these facts? They would probably change their minds.