There are some initiatives that are doing more what you are looking for, vocata and activitypod. These treat activitypub more like a modern email server, that r

There are some initiatives that are doing more what you are looking for, vocata and activitypod. These treat activitypub more like a modern email server, that recieves and sends activities to different federated servers. In a world where those would be used, "apps" would just subscribe to the activities that interest them, and you would be one user that adds apps to your system. You'd be able to have something closer to google suite, where your calendar app can display notes from your note app, or your notes app could display shared notes, comments etc. Today though, most of the big activitypub players decided to implement their own activitypub server and don't really follow the protocol to the letter, so unfortunately it's not as interoperable as it could be. You also need to have an account for each service, which is unfortunate.