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Not everyone can afford self driving cars, and immediately making a thing as widespread a thing as driving cars illegal would have disastrous consequences.
This would result in hundreds of thousands of illegal machines on the road and would a legal and infrastructure nightmare. Most people also would not be able to simply go buy a new car, and even more would still not have paid off their now illegal car.
This idea would take decades to roll out
Because of the reasons others have noted, it would be better to ban the sale of human driven cars, once the market is sufficiently broad.
There are many unanswerable moral debates as to what a self-driving car should do in certain situations and an individual may disagree with the morals programmed into the car.
It would reduce accidents
Removing every car in use would also do that. Unless every single car is immediately replaced by self-driving cars, it would not make sense to make it illegal immediately.