Well, there will always exist people who believe that their route is better than the Google Maps route. Some of them will even be correct!
The other thing is that users just HATE typing in addresses on smartphones. It's like the worst-case scenario for smartphone data entry -- lots of different fields, and words that are not in the dictionary/don't autocorrect well. So Uber (and Lyft, and (ahem) Flywheel (which I work for)) do not require that a user input a destination.
It seems like a flaw in the Uber app that the customer can't just provide an intersection or an address, and the driver gets immediate turn-by-turn navigation. Yes, Uber still needs to blind the driver to destination until pickup, but it's not a huge obstacle.