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.

Maybe they’ve figured out that the level of automation they’d need exists well beyond any runway they could hope to have. More damning, they may have realized that even without drivers, their business model is not profitable.