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?
A unified API and a single login, are two separate things.
A single federated authentication could be a good idea. But the various federated services are different enough that they should have different APIs.
OP is just asking why they a third party mastodon app can’t login to a Lemmy or kbin server, which is a valid question.
From an authentication standpoint there’s no reason for their auth flows to be at all different or use different endpoints relative to their domain.
The returned profile or account object might have different fields which could cause an app to crash, but there’s no reason for every fediverse app to not use some of the same basic schemas and endpoints.