Hijack is the wrong word. Maybe “loot” would be better. Blow out its tires or force it off the road, open it, take the stuff inside.
Why do you think it would be easier to hijack or loot an autonomous truck?
Today, most truck hijackings are a result of collaboration between the human driver and the hijacker.
How does a hijacker collaborate with a machine?
Massive automation will be coming to jails as well, greatly lowering their cost. There won’t be as many criminals loose anymore. While that might be good, it will also be very bad as governments warehouse a far greater percentage of the population, particularly “deplorables” that annoy the government.