They can, but nobody wants a ride except drunks. Which is one reason why taxis look so bad: when a significant portion of your clients are drunk (throwing up in the back, peeing on the seats and all the other things they do) you don't want a nice car. Nice taxis don't work those shifts. If there is a big shared car market (I doubt it) you will see cars for different times of the day as a profile for potential drunk rider.