Did you forget the ./s or something? Lemmy itself is developed on GitHub, as are plenty of other “valuable” open source projects. To pretend nothing of value is built there is putting your head in the sand.
If you’re developing software on GitHub you have a chance at getting some useful feedback, bug reports and maybe even PRs. Like it or not, the network effect is real.
That’s perfectly fine for some things, but for most people letting their browser choice dictate what sites they use is backwards