I'm not sure why you would assume that? It's not something you run on your computer, it's still a website (or set of websites). Admins of your email can also read your email, if they want, and even with gmail in the mix it's probably one of the most "federated" systems ever built.
> I would certainly assume that in 2022 it would built using encryption for the parts that are private, and aren't DM's private? Why would admins be able to read them? Is there a justification for that?
They could potentially be encrypted at rest, in the database, but that doesn't really help much. The owner of the site would have the keys to decrypt them, and on smaller sites it's very unlikely that there'd be any real chain of custody involved.
If you've ever sent a DM on a forum did you think that was encrypted? It wasn't. Or twitter or facebook for that matter. It's not really practical for any data stored on a central server to be encrypted in a way that irrevocably prevents the owner of the service from accessing it.