there is a protocol used by HubZilla called zot, it allows for true nomadic identities. where you can even login to your account from any instance.

I don’t think ActivityPub (Protocol that links all mastodon, Pixelfed, lemmy, peertube instances) developpers have any interests in implementing that, or it isn’t so high in their priorities, also to note that ActivityPub developpment has stagnated since its early releases.