The backstory that seems to have been forgotten here is that self driving cars are still a money bleeding endeavor. It's why Uber sold its own self driving division and it's something Cruise has said in the past still needs to be figured out.
Uber has pivoted to doing partnerships with any self driving player that is looking for a customer base. Waymo doesn't have the tech nor the desire to figure out logistics to compete with Motional or Cartken in the delivery space, for example. None of these players can instantaneously ramp up to millions of vehicles on the road; they physically don't have enough hardware and the age of money burning for growth at all costs is behind us. I can't imagine Waymo has bigger utilization than Bolt, or even Alto.
The way I see it, Uber is more like McDonalds: "easy" to copy from conceptual perspective but also a globally recognizable brand with an undeniably strong customer acquisition arm.
Waymo still has a lot to prove to themselves in terms of ROI. Customer acquisition would be even more expenses on top of the already expensive tech, partnering with Uber to offload these costs to a proven customer acquisition player makes sense for them.