Is there a reason why all the services, that use the ActivityPub protocol don't have a unified API?

None of the mastodon apps allow me to log in with a lemmy/kbin account.

Also none of the lemmy apps allow me to log in with a kbin account.

Even though kbin has both mastodon (microblogging) and lemmy (threads, communities) functionality.

Also, Pixelfed recently introduced "login with Mastodon", but all it really does is just create a new user on it's instance and copy over the mastodon followers and profile info.

Why can't we just have one account to rule them all?