It's mostly tradition rather than a hard requirement. Go has long supported vanity import paths: https://pkg.go.dev/cmd/go#hdr-Remote_import_paths

For example, we use Hugo to provide independent Go package URLs even though the code is hosted on GitHub. That makes migrating away from GitHub trivial if we ever choose to do so (Repo: https://github.com/foundata/hugo-theme-govanity; Example: https://golang.foundata.com/hugo-theme-dev/). Usage works as expected:

  go get golang.foundata.com/hugo-theme-dev

Edit: Formatting

There is nothing tying Go to GitHub.

Not at all. It can grab git repos (as well as work with other VCSs). There's just a lot of stuff on GitHub, hence your impression.