There was a influential internal white paper about not being a "tech island" that drove open-sourcing. The point was that by having its own tech stack Google would eventually be left behind and have a harder time recruiting.
Not sure if the message is still believed as strongly.
The message is pretty well understood - the only difference is that the monorepo (think of it as a service in and of itself) and its associated tooling do get seen as "Google-specific."
Not sure if the message is still believed as strongly.