The reason is federation. People actually happen to be pretty good at holding each other accountable and self-regulating when there isn't a central authority deciding what is and isn't acceptable. Bad actors naturally gravitate toward the instances that welcome them, and then the rest of us defederate from those instances to maintain the peace. Those bad instances then stagnate or fizzle out from the inactivity.
On centralized social media, what stays and goes isn't dictated by the community but a handful of people at the top, and troublemakers are often given a bigger platform than they would have had otherwise.
Mastadon is a lot bigger and older than Lemmy is, and yet it still has the same vibe as this place.
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