Thankfully, some individual instances (like awful!) seem to get it, but for the most part the poison is already baked in, and it’s hard to unbake a cake and begin again.

I think the biggest problem is either the lack of active moderation or, if present, moderators which are too lenient. Not that I blame anyone who thinks that removing the fifteenth racist asshat for the day is not the best use of their time, but the best communities are the ones that to make the effort to keep it clean.

This has been true since the days of Usenet. The good groups were completely moderated to the point where some person had to manually approve every single posted article. It worked (as long as the mods weren't racist asshats themselves, which is a different problem), but in contrast, almost the entire alt. hierarchy was an unmoderated cesspit and to anyone who doesn't know how that turned out long-term: good for you.

Luckily I think we are seeing a rise in moderated communities again. After Usenet and dedicated forums it somehow fell out of fashion (with 4chan and Reddit being the pinnacle of using but muh free speech! to give bigots a platform). Maybe it's confirmation bias, but I do see many fedi instances who have stricter rules again and seem to enforce them in an attempt to create welcoming communities for everyone. I hope this trend continues.