Why is catfight such a big fetish ? Now this is a tough one. I can see many reasons.
There is a sadistic side to it : loving to see somebody in a difficult position. If the "somebody" is someone from the other sex (or yours if you're gay or lesbian ) it can become very sexy. There is usually little clothes(or clothes ripping, which is just as good) involved: sexy. Hard breathing (sexy) moans and grins (sexy) Bodies rubbing one against another (sexy).
there can be the feeling of domination or superiority for the man who sees women fighting clumsily and without any technique or form.
There can be the idea or fantasy that the girls are fighting for you and that, if they are willing to endure such pain and effort, the sex is going to be good afterward.
Those girls must be feisty (sexy).
But, honestly, I can't pin just one of these reasons. There is certainly many more. It is not necesseraly the same for everybody. That would acount for the fact that some guys like to see even matches, other prefer domination, some love to see women fighting in their clothes, other prefer them in bikini or nude. Some love lactating fght, or toe wrestling, or sexfight hwere I, for instance, don't care for any of those.
I think this is a very interesting topic.