It's just a very capital-intensive business with long turnaround times on new hardware designs. It would take not only a lot of investment, but also a large delay on Google's part to get their own car out the door.
It's also requires a lot of know-how, that takes long time to build up.
Building electrical cars removes some of the difference because the traditional drive train is a large chunk of engineering. (Hint, China can't design good engines yet, so they are buying companies with people that do.)
If I remember correctly ABB patched together a electric drive train technology demo car 20+ years ago including a drive by wire system with electric steering and brakes. They probably mostly did it to show off their high voltage pmm controllers.
