That aside, both the passenger and the driver are aware of the optimal route
The passenger is aware of what his smartphone thinks is the optimal route.
Consider going from the Spokane airport to the Lakeside area: Google Maps routes you via I-90 and the Maple Street Bridge, but during rush hour, this is a wonderful place to run up the meter, with delays of 10-30 minutes on a 30-minute trip. Going via the Sunset Highway instead avoids much of the traffic (and cuts a quarter-mile off the meter), but to someone who's not a local, it looks like you're being dragged off into the middle of nowhere to be mugged, or at least ripped off on the taxi fare.
Now, as someone from out of town, how are you going to judge if the driver is telling you the truth about why he's going somewhere your smartphone doesn't want him to?