[1] https://lore.kernel.org/lkml/CAHk-=wjbtip559HcMG9VQLGPmkurh5...
https://github.com/torvalds/linux/pull/17#issuecomment-56546...
Without involving github at all, you can use git directly to push branches to a remote repository and then have it be merged by the maintainers there, which leads to a workflow that's pretty similar to the one you have with github.
Instead, Linux is being worked on by exporting the (git) patches and sharing them on a mailing list, effectively not using a significant feature of git and leading to a very different workflow than the one everyone else is using (github users and others alike), so I still find the question legitimate.
Once configured, you can send a series of patches to the mailing list with a single git command, and with just another command apply a series of patches from the mailing list. It's actually quite efficient.
In addition, email is much easier to search and look at after the fact than a website. Especially with GitHub I have seen "these commits do not exist anywhere" errors way too often.
Related to this, email is easier to mirror. This causes a very problematic lock-in due to how much content is archived on the git forge that you've chosen.
Anyway, everything wondered where "Pull Requests" come from? Literally an E-Mail saying "please pull my branch ... at ....". Git has special tools to create and apply mails.
GitHub copied most of the LKML/kernel development flow and turned it I to a web UI. That's nice, but the text workflow is often faster and easier. And it works. And it is decentralized (a key requirement).
So yes. It is not broken. And GitHub would break the decentralized nature to some extent.