Nox - the logic that something has gone on for a long time, and is therefore acceptable, is simply not valid.
Somewhere in the world, as I type this, a man will be killing another man, and then eatring his flesh. This has gone on since the Stone Age, therefore, according to your logic, it is perfectly acceptable.
It would be nice to think that as a civilised and sophisticated society, we have left behind some of the barberous practices of our ancesters - we no longer pit people against each other in arenas to fight with weapons, we no longer bait bears, or allow dog fights. All these things are a matter of hisotry, but society feels they are no longer acceptable.
Pandering to base instincts simply because people have them, and there is money to be made, is not an acceptable reason for boxing - any more than it would be for gladiatorial combat, or indeed dog or cock fighting.
Dogs and cockrels follow their instincts, so what's the problem in people being entertainied by them doing so, in organised contests? That is the logic you are using - it's wrong, dogs, cockrels, men, violence for entertainment is barbaric.