Other than a decidedly not-Uber-like experience of throwing ads at a captive rider, I really don't see a reason that Google would be interested in that space.
Google wishes it wasn't 95+% an ad company. It's a highly profitable space, but it leaves them vulnerable in the long term.
Google is entering into the delivery space (Google Shopping Express) and into self-driving cars, both which are similar to or complementary to Uber's business model.
Commercial trucking is the first market that will be overturned by self-driving cars. More predictable routes, do not have to deal with passenger misbehavior, higher total profits to absorb early-on technology costs associated with creating the self-driving vehicle (LIDAR costs and whatnot).
Google has other realms that could benefit from self-driving cars, not just ride-sharing/taxi. Why have separate fleets of vehicles if you can have one that does taxi services, shopping deliveries, and data collection (street view) at the same time?
