Linux development is also centralized. Instead of Github they use an email list that has patches sent to it. If that goes down, your change isn't going into Linux today.
A mailing list can go down and nothing would happen. The main point is to post patches to the maintainer. The mailing list is for a public record of things.
The only centralised thing is repo hosting on kernel.org. And that isn't the only official place, you can get the repo published on googlesource or GitHub, so it isn't exactly central enough.