Sure, and it'd be kinda neat if I could, say, use DNS records in a way to allow specific servers to post as my identity, thus, both proving that I'm me (from the [email protected] sense), but also the server can post as me (maybe what I have to put in the record is me countersigning their sign-as-me key).
We don't have that yet. It'd be nice if we do someday.
But that's the thing, I as an end user don't want to and shouldn't have to. Hosting my own instance is a pain in the ass that introduces needing a server to run it on, OS maintenance / updates, Mastodon instance maintenance / updates, paying for everything, ... It's a huge waste of my time and energy when decoupling identity from content solves it much more smoothly, for a much wider audience. If the goal of Mastodon isn't to attract that sort of audience, that's a separate issue, but that doesn't seem to be it from what I can see. Identity and content can both be federated without tying them together.