There was actually an interesting paper written about this recently: http://john-joseph-horton.com/papers/uber_price.pdf

Essentially, when Uber raises fares it increases earnings in the short term, but not long term, because more drivers enter the system and push their earnings back down as each driver gets fewer rides.

Uber could solve this problem by limiting the number of drivers. But that of course turns some underpaid Uber drivers into unemployed ex-Uber drivers. Which doesn't sound like a desirable outcome either.