A higher percentage of people drive and own a car in California than most places on earth

This is one reason why Google would want California to be it's test case for building and selling driverless Cars. Other companies might think this way too, especially withe the kinds of laws California has in place regarding all motor vehicles.

Why do more people drive cars in California than other places?

The first reason is that California is a very big place sort of like Texas and Alaska. So, if you don't drive often you can't accomplish much in a place like California where everything is spread out. When I was growing up it was very normal for people to drive 1 to 3 hours to work and from work. Some people still live like this so they can have a remote wilderness lifestyle for their children and work in a big city to pay the bills. This is possible by living in places like Big Bear, Lake Arrowhead, Idylwild, the deserts, the beaches etc. where a higher or more desirable quality of life for one's family might be achieved.

However, as the price of gas fluctuates and salaries fluctuate all these things change over time. So, what is good and possible in one era might not be useful in another era.

But, basically, people drive so they can have a better lifetyle, make more money and still have a higher quality of life. This is why so many drive so far in California every day of a working week. Then on weekends people often drive hundreds of miles.

Another interesting fact about California is 10 percent of all new cars bought in California are either hybrids or all electrics like Teslas.