Partnering with an existing car company helps a lot. They know how to build vehicles at scale and they also can provide a lot of support when it comes to interfacing with the control systems for the car. Controlling the car (engine, doors, wheels, wipers, lights, etc) is a surprisingly difficult task on its own. It requires a lot of control systems knowledge, a complex test infrastructure, and a lot of specialized talent that's probably hard to get. It could've been seen as too difficult to take on in addition to figuring out how to actually do all the self driving specific tasks (mapping, planning, detection, etc.).

> They know how to build vehicles at scale

The actual car companies also outsource lot of stuff to other manufactures e.g lights, wires etc. I would imagine understanding and maintaining those relations woulds also be a hassle for Google instead its just better a car companies does all that.