While those do exist (and plenty of them), many people avoid them precisely because content moderation isn't possible or regularly done in those communities. From people who prefer not to sift through egregious amounts of bigotry/spam, to people who are at very real risk of having their physical safety or livelihoods damaged by online abuse (e.g. children), there are plenty of reasons to avoid those communities--reasons why those platforms are often ghost towns or toxic echo chambers.

Like, this isn't a "think of the children" argument that those communities shouldn't be allowed to exist. Of course they should. But don't be surprised when people prefer more centralized and "censored" (policed/moderated) communities. For the vast majority of people, that is a feature, not a bug--and they tend to realize that and boomerang back to Twitter etc. when they try out decentralized/anticensorship-oriented alternatives.