You're right, it does not structurally need to be centralized and there are small trusted networks that are volunteer-moderated. They are a minority of what peo

You're right, it does not structurally need to be centralized and there are small trusted networks that are volunteer-moderated. They are a minority of what people actually spend time on when they use social networks these days though. The strong economies of scale involved in content policing (eg, investment in ML porn classifiers, moderation guidelines enforcement, dealing with the legal questions associated with kicking-out of a political person calling for this or that violent act) make it economically advantageous for centralized networks to pursue a scale-up strategy.