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.
If an instance doesn’t want to grow they turn off new sign-ups, some growth might help instances stay afloat, as not only it might help with more donations, but also signal to the maintainers that there is appreciation for their work.