They’re valued on growth, so if they decide to stop growing their valuation will plummet.
If they raise prices then quantity of rides will go down. If quantity of rides goes down, it becomes harder to earn a living as a driver. If it is harder to earn a living as a driver, there are fewer drivers. If there are fewer drivers, then the cost of drivers goes up.
Not to mention the fact that if uber becomes 10% more expensive a large chunk of those customers will just go to lyft.