Parts have been. Sourcegraph is basically the code search post built by ex-Googlers originally. Bazel is the open source build tool. Sadly, most of these things require major work to set up yourself and manage, but there's an alternate present where Google built a true competitor to GitHub and integrated their tooling directly into it.

Building tools for others is a competency that is under rewarded at Google. They would never.

I've published my Google proprietary stuff (when we decided to open source it) on GitLab, but they wouldn't let me do it on GitHub.