Harassment is illegal as is conspiracy to commit harassment. Since we all view harassment to be illegal, a case where there is evidence that a person "sic their followers" to commit harassment should be sent to the legal system, or if that fails we should make a law so that future cases will be addressed through the legal system.

I think the reason we don't is simply because we want to allow speech that result in "bad" people getting harassed but not "good" people, and since different political sides define good and bad differently, what we get is platforms making the decision for us. Personally I would prefer a law instead.