If I read between the lines on the software design for #ActivityPub, I don't think there are actually supposed to _be_ servers per se, or more precisely that servers are supposed to be very simple passthroughs that have some forwarding/processing logic?
If so then this makes some sense, since when AP was being written was a big time for BaaS (Backend as a Service; https://en.wikipedia.org/wiki/Backend_as_a_service ) and that influenced a lot of ideas.