Cars and parking take up too much space in cities. They require too much investment to manufacture. They require too much hardware, land, etc., for upkeep (fueling, repairs). They are too damaging to the environment (air, water, landfills (tires and all the rest), etc.).
Much of what people use cars for is for getting to work in a reasonable amount of time. Rapid transit and dependable wide-ranging bus-service is needed. But that's not enough.
My morning commute is about 25 minutes door to door by car. By bus/train it would take at least 2 hours. By bicycle it would take about 75 minutes (would have to stay off the highway). But by 25 MPH scooter/segway/moped it might take 30-45 minutes if there were a legal pathway similar to my driving route.
Having everyone take Uber isn't a solution either - it's too expensive and it won't reduce the number of cars on the road during peak hours (which would naturally increase as cities continue to grow).
There's nothing about my commute that requires that I take it in a 3500 pound vehicle. It's hard to believe that in 50 years we won't have much more efficient ways to commute.
Yeah, semi-autonomous is coming and it's going to help in a vast majority of cases. But Atrios is right that the people selling the future as being completely autonomous are dreaming and it's not really clear why customers would want such a thing anyway.