I don't think it's either a feature or a bug but simply something the goals of the project don't care about.
ActivityPub is completely focused on instances. It's all about the content that instances are putting out.
To talk about migrating from one to another is sort of speaking gibberish to the fundamental philosophy of ActivityPub since users are such second-class entities in the system.
I think it was a choice made by server-focused people making server-focused design choices.