It's because they allow social conflicts. When you allow people to bitch at each other you also allow them to resolve social conflict. That is you'll get into an argument with someone, you'll call each other a bunch of names, have a bit of a back and forth, if you go to far people in your vicinity will start to bitch at you, then you'll break up and that's that. Only if it becomes a major disturbance do mods step in.
Meanwhile in other places mods micromanage conversation and especially conflict. If someone says something retarded and you call them a retard, mods immediately step in and break it up. Nothing gets resolved, everything is left to fester. And not only that. People quickly learn that the way to get around that is to become extremely passive aggressive, adopt fake politeness, and get in good with the moderation staff so they take your side (and the mods get involved in every bitchfight, even if it's just to break it up, they inevitable take sides as a consequence).
Oh and if someone is a really awful person it's a lot easier for them to hang around in a place where they can use the mod team as an enabling shield against mockery and humiliation. People who are to anxious and can't handle online conversation will leave places like Kiwi Farms.
All of that combined and places like this become a lot more chill than conflict averse communities (most of the rest of the internet by now).