Because then the only thing you're relying on the centralized closed-source company for is actual git hosting. This is a boring problem that dozens of other services solve well, and thousands of other services (like S3) solve poorly in a pinch. If you can migrate the interesting part of GitHub -- issue tracking, PRs, wikis, etc. -- to another provider, and GitHub just holds your data, then you're no longer locked in.

If you don't build it and you wait for the centralized closed-source company to, then you're putting your project even more in their hands.