From Waymo's perspective it is certainly desirable to have someone else paying for the cars.
The problem is that when others spend on it, Waymo has more leverage over them if the cars are only going to work with the Waymo driver. Perfect vendor lock-in on assets worth $150k each.
What's in it for Uber? Perhaps just a more favorable & longer term deal, but if so, they're essentially handing over the future of their business to Waymo.