Probably network effects, integration, just about everyone knows how to use it, high switching cost, the fact that it’s mostly good enough, etc.

Even if you switch away from GitHub, you’re probably going to use it for something else anyway. There will be some dependency or something that’s still on there or you might use it for discovery. Then you have to spend time and money migrating to something else for what? Comparable features and interface? Maybe avoid a few minutes of downtime every month? Migrating a team might cost the equivalent of years of downtime. Or host it yourself to spend more of your own time and maybe get a worse result?

Really, what does downtime cost anyway when you are using a distributed version control system? It’s not like you don’t have access to the code. You might not have access to issues or CI for a few minutes. So what? It’s annoying, but is switching really worth it?

And what else is out there? Gitlab? Gitea? BTW, that’s the dumbest name ever for a product. I wouldn’t use it just so I don’t have to say that word. Are either of those enough of an improvement to justify the time? Or host Forgejo? Now you’re in the hosting business and only have yourself to blame when it inevitably goes down.

It’s easier to just use GitHub even if GitHub isn’t great.