As others have pointed out, Google could scoop up tons of riders in a heartbeat, by announcing a ride share platform publicly (which would get tons of news coverage) and by advertising the feature within Google Maps, Waze, etc.
Google only needs to put together a one-sided marketplace, where Uber/Lyft had to build a two-sided marketplace. And Google already has a connection to billions (at least hundreds of millions) of potential riders.
It would also need to invest in the vehicles and the technology and the maintenance and the garage to store them in and pay the insurance on the vehicles.
All this to try to compete with Uber (which isn't profitable) in a few areas where the legal, political and climate ("we have cars in Chicago but they only run from March 1st to November 1st") options are favorable.
The marketplace is not the biggest issue that Google would need to overcome.
I don't know a lot of people who love Uber, but I know a lot of people who are only ever going to use one ride sharing app. If Waymo's app is limited to a 180 sq. mile territory and only half of their trips are in that territory, they won't bother. The first time the Waymo app tells them they're going outside their territory, it will be back to Uber forever.
And it isn't the lawyers.
It's the network of riders.
Waymo promised to 10x their scale this year.
Waymo could saturate the territory with empty vehicles waiting for riders on the Waymo app, or it can tap into a fraction of Uber's volume.