My guess is that all their code is in their monorepo (google3), and they don't want to set up a tool to sync it to a public git repo, so the easiest way is just to have someone create a tarball on demand.
But google already built a tool to do exactly that: https://github.com/google/copybara
But google already built a tool to do exactly that: https://github.com/google/copybara