When I read comments on Reddit I often see a lot of frustrated and burned out people with short tempers who might not have someone IRL who will listen to them vent. Like you said it makes sense, but that doesn't make it any better.
What makes me optimistic about decentralized social media is that the communities are (hopefully) small and varied enough where mods and admins can keep an eye on everything much easier, and step in an say "Hey, you're not being nice right now" when someone isn't. It's one thing for communities to have rules, but you can't make enough rules to maintain a culture of amicability. We ultimately need humans for that.