If you can say "easily" for this, you can't have tried actually importing the OpenBSD CVS repository into git. cvs2gitdump doesn't do too bad of a job, but doesn't attempt tags and branches. I haven't found another conversion tool that gets anywhere close.

Your analogy is invalid. A correct analogy is that Facebook owns your photos and posts the way github owns what you push.

No, Facebook and Reddit, big as they are, don't make their own browsers and are still individually minorities in the grand scheme of the world wide web.

Git is used by a comparatively small captive audience; most git users are invested in GitHub. MS is well positioned to run their "embrace, extend, extinguish" play if they wanted to.