Assuming that Uber and Lyft are barely profitable, they don't have extra cash to pay the drivers extra, therefore the only source of an increase in payment must come from an increase in ride fare (thus an increase in the percentage going to the drivers).
Assuming that Uber and Lyft receive a percentage of each ride's fare, then if they could increase prices it would increase the amount they receive per ride.
So if it's in their interest to increase the fare, which increases the amount per ride paid to the driver AND themselves, why won't they do it? Greed? No, not if they would make more the other way. Politics? Maybe.
More likely it's just tradeoffs: Lower fares mean less revenue per ride but more rides. On the other hand, higher fares mean more revenue per ride but fewer rides. Instead of pulling out of the city, why not just increase the fare price in that city? Because say that the fare for a ride was $200 and nobody would pay it? Then Uber and Lyft would still have to pay the driver $15.57 an hour. I'm guessing that's the issue: a requirement to pay a driver a certain amount even if no one is willing to pay for a ride at those prices.
I've spoken to many Uber drivers. Most I see do it as a hobby. One told me it was better than watching TV. Another few were retirees who just wanted something to do. Another few said they loved the flexibility: if they didn't feel like working one day, they didn't. The system works for that type of person. If it doesn't work for someone looking for a $15.57/hr job, perhaps they should look elsewhere.