Granted, but a human guard nevertheless presents a problem for robbers.

Do you take the guard hostage? Do you kill them? Will they themselves be armed? Will they put up a fight, training or no training? If you try aggressively to bring their vehicle to a halt, will they concede immediately (as an autonomous vehicle likely would), or will they try ramming your vehicle?

A human guard is an unknown quantity, and any crimes you commit against their person are in a different league to the theft of the cargo.