As such, Uber may have decided that they were going to optimise their display advertising solely based on installs, and not a lower-funnel action like rides booked, because they didn't expect to get much data for the latter.

Uber is NOT there to get rides. Uber's business model is a scam designed to skim money from investors and drivers alike.

That money goes into the pockets of the principals.

The reason that Uber can sell cheaper rides is that the investors are subsidising it, and the drivers are subsidising it. Once the investors and drivers wake up, Uber will collapse. In the meantime, however, the company principals will do extremely nicely, thank you.

There's a reason that taxis are more expensive, they actually have to break even or make a real profit to survive.