The problem is that moderation doesn't scale. Small communities and small instances will nearly always be better at moderation than a large platform, because they can rely on a small number of individuals to get the job done - individuals who are more likely to be consistent, more likely to pay attention to community reactions to posts, and more likely to care in general about what they're doing.
It's difficult for me to think of any large platform (Facebook, Github, Twitter, Reddit, Steam, Amazon, etc...) where moderation and general content quality is better than their smaller counterparts (Mastadon, Itch.io, Gitlab, self-hosted forums, etc...).
Large platforms also become large attack vectors. The same benefits of centralization for users - (content discovery, single sources of truth, and so on) make it very cost-effective and efficient to spam and harass users. With a decentralized system, spammers are less likely to care about whatever small community you're hosting. It's not impossible for them to crawl around the internet spamming everyone, but the cost of doing so is a lot higher than targeting a single platform.
So centralization inevitably leads to large platforms because the market for these platforms is winner-take-all, and it almost a kind of pseudo physical law that large platforms can not be moderated well. I don't want to be absolutist about it, but I'm really having a hard time thinking of even a single exception to that rule.
Maybe Wikipedia, but I'm even kind of doubtful about that - and most of Wikipedia's quality moderation comes from a group of people who are completely obsessed with the project. That mindset also doesn't scale to Twitter/Facebook sizes.
