Driverless trucks will be better protected, not worse. You can lock the vehicle much better if there's no driver who needs access to it. Cameras everywhere will mean that the control center sees people approaching the truck much earlier than drivers at the moment (who won't see it at all if you keep in the blind spot). It's the same as robbing an ATM. Theoretically also unattended but well enough secured that few people do it. The insurance premium for cash in an ATM is lower than in a bank branch where the teller can access it. A driver doesn't provide a lot of protection against serious theft, video surveillance could deter more people.