It's because vim has been around looooong before GitHub, and was only moved to GitHub (relatively) recently. Bram is old-school and probably just uses the project management strategies he always has. Most vim development takes place on the mailing list anyway. I'm not saying it's great how he does it, but it's not super realistic to expect a project that's been around since 1991 to fit the normal GitHub workflows.
There are other good reasons to use neovim, but this one is anachronistic, in my opinion.
Why would you want to adapt your workflow to fit one certain VCS when your project is basically driven by you as the BDFL?
Now I see, yeah, I agree with you.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.