I used to think running my own server for blog stuff was a waste as well. But it’s incredible what can be done with local mini server running Ubuntu and cloud flare tunnels.
you generally need a stable IP, and a firewall that'll let people initiate connections from outside your house
That's a bit besides the point though. I'm not saying hosting ActivityPub yourself seems unnecessary, I'm saying that ActivityPub itself seems unnecessary (or at least not worth the costs and overhead).