The reason is that it's a technically difficult problem, and thus requires investment to solve (and the most profitable/successful platforms don't want to reduce the amount of information they have about their users).

Fortunately there is work being done in this area by Free Software projects. In particular, the Matrix team are trying to implement a decentralised reputation system[0] and there's no reason this couldn't be applied to the Fediverse too.

[0] https://matrix.org/blog/2020/10/19/combating-abuse-in-matrix...

Secure Scuttlebutt does exactly this. It is supper effective, and the only social network that I participate in, except for HN.

It's effective because you follow the profiles that you find interesting, when a profile becomes abusive you simply block them from being replicated by your machine. Folks can have as many profiles/feeds as they like, so you can separate out portions of your life that others might find distasteful, or boring.