But there would be no point to a federated network if every node were required to accept traffic from all other nodes. That's just centralization with extra steps.. and centralized social media is what most people who move to federated networks are trying to avoid, to say nothing of the cost burden on smaller instances to handle all of that traffic. A lot of people block the big instances as well for similar reasons.

(Google Groups was to Usenet what Threads is to ActivityPub, and it got collectively blocked by all Usenet servers for spam, then decided to shut down.)