Is there any fundamental reason why no tool offers a federated alternative to GitHub? I mean, the distributed nature of git is ripe for federation, but why aren't issues, releases and pull requests federated?

Maybe even some deeper integration with git itself, such as seeing new remotes on federated servers and fetching them.

This may sound redundant to GitHub, which currently centralizes everything, but different projects and companies have their reasons to not use a single external tool for this. As an example, CERN's Open Hardware Repository (https://www.ohwr.org/) would be a perfect fit for this, and other laboratories and companies could integrate with them.