I suspect this kind of problem is unavoidable when a large fraction of the world uses a centralized many-to-many communication platform. Moderation is always going to be a cost center for such a platform and there's a big gap between good enough moderation to prevent a mass exodus and what most of us would likely call good . Using decentralized tools might improve the situation.
Without centralization it's just some kind of trust network, and since the messages are open it's a much easier problem to filter away unwanted material than to deal with things like e-mail spam.
The reason people don't is simply that they want power over political discourse, so they accept what part of these problems are problems for them and are then happy for the other problems.