that is because of replies being the only "reinforcement" (gamification of behavior) you get on 4chan.
There is no point system or similar, what keeps a thread a float is replies, and some people like having many responses under their posts.
This means that creating inflammatory posts, sometimes is the only way to get attention, or worse, when you actually want information and you post in a way that would get attention otherwise it would be ignored.
Identity politics usually gives plenty of reactions. Just like calling something bad gets more reactions that call something good. Want opinion on something you want o buy? start a thread calling it shit, and wonder why there are so many bad threads there.