Blaze has been: https://bazel.build/

Forge and Piper are built on Google's internal tech stack and designed for Google's production infrastructure, so open sourcing them would be a very big project. I think it would be a lot more likely for them to be offered as a service -- and that might be more useful to users anyway, since you'd be able to share resources with everyone else doing builds, rather than try to get your own cluster running which might sit idle a lot of the time. Of course, there are privacy issues, etc.

(Disclaimer: I'm purely speculating. I left Google over four years ago, and have no idea what the tools people are up to today.)

Because very few people have a need to support a billion-LOC monorepo on which 30K engineers make tens of thousands of commits daily. That's where this system shines.

For smaller projects, Git+Bazel (open source, non-distributed version of Blaze) works fine if you're working with C++, and other build systems work OK as well, if you're working with other languages.