I can't help but notice most (all I've seen anyway) of the federated projects are hosted on GitHub. GitLab is also not federated, but can be self hosted and has at least discussed it.
As a long time user of both, I vastly prefer gitlab. Not only can I contribute code to gitlab (I have), I can also run my own gitlab (I do) and there are a huge number of projects using gitlab. It's not like that hard to find projects, or their gift repository because some are hosted on github and some on gitlab. Project just links to their repo, or you use a search engine... I feel your arguments apply to the fediverse as well?
I don't get what benefit hosting your own git brings to be honest
Just another level of backup. Personally I tend to have:
A copy of my repo on my dev machine
A copy on a self hosted git server. Currently I'm using gitbucket though.
A copy on GitHub.
This way I should always have 2 copies of my code that's accessable at all times. So that there's very slim chance that I'll lose my code, even temporarily.