The key functionality of the major tech platforms is to convey data to other people. Almost none of the data we give them is private or proprietary in the sense you’re suggesting. We only give it to them because we want e.g. our friends to have it. Now it is unfortunate that the service provider also gets it in the process, but that’s more an issue of end to end encryption than centralization per se. We can have highly centralized yet end to end encrypted platforms, like WhatsApp and Signal. We can also have highly decentralized platforms which are panopticons, like cryptocurrencies.

One of the main reasons we “hire” the platform companies to convey our data to other people rather than opening TCP sockets to each other directly is precisely async delivery. No one wants to use a social network where we can only see each other’s content if we’re using our laptops at the same time. Just make a phone call at that point.

Now maybe you could ask for federated, open standard queueing mechanisms. But we have one of those! It’s called SMTP, and it’s not really up to the needs of modern social applications. And maybe we could do something about that. But it’s also in the nature of federated open standards that are widely deployed to ossify. The other service that Facebook and Twitter are doing for you besides pub/sub is having the coordination and agency to update their own deployments over time, something that e.g. the set of all relevant email operators does not have.