Humans* aren't built for decentralization. We can only keep so many unique names in our head a time, only so many connections. I don't have any papers to link to on this, but I think it's safe to say that's how things are - if not we wouldn't have created cities, national identities, and of course, newsgroups, message boards, centralized package managers and distributions, and yes, source code repositories.
There is something nice about knowing, with a high degree of certainty, that any given open source project (or any thing) is in one of a few places, and if I don't know where that is I can ask for help to find those places by their names.
Many humans are apparently not, but I had a much easier time navigating (and contributing to!) projects when each of them had their own website with integrated scm links etc.
GitHub is a grey mass, project branding is lost, the tracker is chaotic for large projects.
It's just Sourceforge, better executed but with the same disadvantages.
Caveat emptor.