Because they’re all different applications. I think the confusion here is between ActivityPub the protocol, and the applications that actually use it. The applications that use AP are all different, they have different data structures, hell mastodon and Lemmy/kbin are completely different at a conceptual level. They just communicate with each other via AP, but once they receive the AP message they convert it into their own data structures and concepts. And you should note that AP is technically a communications protocol, it doesn’t prescribe how stuff should be stored or sorted after an object is communicated between two servers and doesn’t really prescribe a way to browse through the historical activities of a person. These are things implemented by the application you’re using. So it’s not like you could just write an app that combs through all that data available on the fediverse, you’d need an instance that federates with all these places, then an app that uses that instance. Technically feasible but so far nobody’s done it yet, but you can see how some people reply to threads on Lemmy via Mastodon.