By beating people to market with self driving cars. That's the reason Uber is pushing it.
Because deploying Robotaxi fleets will be so capital intensive, no single company is likely to dominate the way Uber has in rideshare. There is enough demand spread out across enough different cities that they won't need to compete directly for a long time.
If they can't maintain their market share in a driverless car world, it's difficult to imagine how they can justify their current valuation.
https://newsroom.uber.com/san-francisco-your-self-driving-ub...
https://www.theguardian.com/technology/2016/dec/16/uber-self...