There's a lot of reasons, but at least part of the answer is technical. Most Google software is extremely dependent on a massive set of proprietary internal lib

There's a lot of reasons, but at least part of the answer is technical. Most Google software is extremely dependent on a massive set of proprietary internal libraries, and it is so difficult to disentangle them that rewriting the entire thing from scratch is often easier (like they did to create Docker). I'm not sure whether it is possible to provide a contractual lease to an external company granting access to the entire Google codebase, but it certainly has enough challenges they would rather avoid it if they can.