The reason you and I can't share a human driven car right now is because I need it to drive to work and so do you. But I leave for work at 6:30am, usually, and you might leave at 8:30. We could share a self driving car that drops me off and then goes back to pick you up.

It becomes even easier with a bigger pool.

Also, it breaks the driver/car connection since you are no longer a driver.

Finally, the real reason. Without a cab driver to pay and tip, cabs are actually cheaper than ownership. This is Uber's end game.