It could be that they've realised the 'self-driving' part of 'self-driving cars' is only half the story - they'd also need to build cars. That's really hard to do at scale. Just look at the Tesla Model 3. Tesla has been making cars for more than a decade, and they're really struggling to scale up even with all that experience.

After all, Uber don't need to build the self-driving cars themselves to take advantage of the cost savings from removing the driver. They could just buy them once someone else has solved the hard problems.