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...