Machines also need people to maintain and repair them.
The Simpsons will be proven right again.
“The wars of the future will not be fought on land, or sea. They will be fought in space, or on the tops of very high mountains. And they will be fought not by you, but by robots. And as you go forth today, always remember, that it will be your duty to build and maintain those robots.” - grad speech, Bart graduating from military school.
So you have robot guards along the US/Mexican border. How do you tell the good guy from the bad guy??