Because one near-certainty of the driverless era is that more people will want to drive. That will have wide-ranging policy implications, including the need to modernize the way we finance transportation infrastructure. That, in turn, means more user charges, such as congestion pricing and variable tolls, and especially a vehicle-miles-traveled tax to ensure that drivers bear the cost of road upkeep.
Ceding our roads to computers will be a complicated and chaotic process. It will involve all kinds of strange moral considerations and trade-offs. It could transform our physical environment, our economy and quite possibly our entire way of life. The technology is unquestionably revolutionary. Just don't expect it to be a joyride.