“.why don’t they set up a RF DF “
That could work, but the difference between a “drone” and an RPV is that a drone is able to be programmed to perform a mission without operator input. Military drones tend to operate on some combination of autonomous and remote control. A mission involving going to a grid coordinate, and taking a bunch of pictures could easily be done autonomously. And drones don’t need to be just for governments any more the technology has gotten to a point where it’s not that hard to build a drone, some people do it just for fun.