Admins are legally responsible for anything that gets posted on their server, so they require control. They also put in time and money to keep the server running.
There may be other technologies where all-powerful admins aren't necessary, like peer-to-peer networks. But then everything gets much more complicated. No more opening a website and entering username and password, now users need to install a client first and use a private key. It's not clear who is responsible for moderation. Scaling will be much more difficult. Users don't want to store gigabytes of historical data on their devices, so some servers are still needed.
The way Activitypub and Lemmy works is really a hybrid between normal websites which is a well-known paradigm, and p2p which is still experimental. The p2p part is only done on the server because it would be too complex otherwise.