The reason it's not a fundamental human right is because it implies a right to require someone else to work for you against their own interests.
It implies a right to another person's labor, which is not a right I would recognize; certainly it's a right that is generally rejected in cultures around the world these days.
But on #Fediverse in particular, the ability to join instances with a diversity of policies, and respecting that diversity, objects to that sort of blanket statement about rules to which people must conform as they express themselves.
This platform is built on diversity, not conformance.