It could be solved by the filter but filter would also have a bunch of false positives
It just doesn't have to be spammed enough that advertisers leave the platform and I think that they sort of succeed in doing so.
Think about it, if Facebook shows you AI slop ragebait or any rage-inducing comment from multiple bots designed to farm attention/for malicious purposes in general, and you fall for it and show engagement to it on which it can show you ads, do you think it has incentive to take a stance against such form of spam
I'm not sure that's actually true. It's just that at scale this is still a hard problem that you don't "just" fix by running a simple filter as there will be real people / paying customers getting caught up in the filter and then complain.
Having "high engagement" doesn't really help you if you are optimizing for advertising revenue, bots don't buy things so if your system is clogged up by fake traffic and engagement and ads don't reach the right target group that's just a waste.