Because owning a car is expensive. And owning a self-driving car is likely to be even more expensive. If you can still get the same convenience for a far lower cost by just using a taxi, why would you continue to own a car? (Also, this is something the automotive industry does not want, as it will reduce their sales substantially. This is something that is claimed by self-driving companies which are not also automotive companies, and most people who analyse the economics of self-driving, especially looking at which technologies which are more likely to succeed).