You are assuming that driverless cars will be just as inexpensive as the typical American car today. That's certainly not true today. For example the Google car has $150K in equipment costs. Sure that might go down in time due to more demand but I doubt it will become literally negligible cost wise. Besides that, there is the added cost of software development, and maintenance. And besides that there is money to be made in selling software for these new cars. Software like up to date datasets or more accurate traffic avoidance heuristics.