Plenty of opensource projects are in google3(that frequently gets merged into external)

I think that's an effect, not a cause. It's annoying to open source google3 code because it means untangling dependencies. Meanwhile Android has lots of SOC-specific and other development happening behind the wall. So the open source question seems orthogonal.

Google today has separate repos (android, chrome, chromeos, google3), each with its own build system: Gradle, gyp/ninja, Portage, Blaze. There's hysterical raisins, but I wonder if Google considers it to be a good thing these projects are so different, or a wart they would prefer to fix?