Why is so much Emacs stuff hosted on GitHub these days? It seems antithetical to the spirit of the project as a whole.
As opposed to what? Buying a domain and hosting your own server? To answer your question: Some small programs would probably not exist at all without a service like github.
It's not necessary to buy a domain and host one's own server. For example, there is https://savannah.gnu.org/ , which is where Emacs itself is hosted.