Basically, the software engineering decisions that went into the ActivityPub protocol were really focused on the instance, not the user, being fundamental, for better or worse.
The benefit of the doubt argument might be that they were imagining different instances having different communities, with a sense of home community.
But in the end it does make it like email, where if you had email addresses at gmail and msn, you wouldn't expect the inboxes to synchronize.
There are some people with ideas about how to make what you describe work, but they generally seem to be kind of kludges, trying to make the protocol do something it wasn't really supposed to do.
Sure, and if you can't tell, I'm critical of those design choices :)
But it is what it is. Horse is out of the barn. Now we work with what we have.
@volkris Understood! I was just curious, y’know? :-)