Unresolved issues surrounding Follow activities explores this from years ago, but basically: if you don’t give an object an inbox, it’s not clear where to send the Follow or if the Follow is even supported. It’s not enough to say “just send it to the attributedTo actor’s inbox”, because the attributedTo actor might not understand or support sending you updates about any given object. It’s just architecturally cleaner to give it its own inbox property. And also a followers collection, even if it’s private, to indicate that it can be followed.