Github hasn't centralized all open source code Because git itself is decentralized. If only other social networks let you extract your data so easily.
The source code itself is stored in a decentralized way, sure, but the git repository doesn't include pull request comments, wiki pages and issues. Of course, you can fetch these using the (rate-limited) GitHub API, but it's not quite the same (and other social networks typically let you extract your data in the same way).
I do agree about other social networks though.