Take somebody who has never done a pull request (GitHub-style, not Git style) and has also never created a patch, and then try to teach them how to do both. Which is easier? Creating a patch will win every time.
GitHub has managed to cause a culture around git that there exist problems that don't exist that they can solve, and then their solutions are one-off specific solutions that only work so well if you are willing to use all their other features and you get all your developers and users to use the service as well. You are correct that GitHub is every bit as much a walled garden as Facebook.
It seems the familiarity of GitHub was actually a primary selling point.
> There were multiple reasons as to why I made that decision. One was that GitHub has basically built a social network of open source contributors. That led to various core developers telling me that they were comfortable with GitHub already and they were hoping it would win.
> Lastly, our BDFL prefers GitHub. Since the beginning of this whole decision process, Guido let it be known that he thought GitHub was the best choice.