Technically, it wouldn't work at all. All Google projects are deeply tied down to Google's own tech infrastructure. They would need a complete rewrite to function outside.

Legally, it's difficult to ensure the IP is properly handled. I'd go as far as to say it's so difficult it's impossible.

Financially, there's almost no incentive.

Because if you're an executive that signs off on a project spinning out, and that project then becomes wildly successful, you're going to be blamed for letting that super lucrative project "escape".