I think you're underplaying the difficulty of what Uber has built. Uber is an operations-heavy business. Determining pricing strategies, handling incidents, and working with local governments is at the core of its service. Google has traditionally shied away from businesses that require big operational investments in real-world assets the way that, say, Amazon or Airbnb have. These take time to build properly, and is a big piece of the puzzle that I can't see Waymo wanting to take on. It makes sense for them to want to partner, especially now that Uber has spun out Aurora.