Those things run on batteries, which have a pretty short lifespan. Also, there are plenty of patriots that know how to program robots and would happy to take them over.

Aside from that, any armor effective at stopping even a pistol round would drastically reduce how long they can operate, radio jammers can interfere with their command and control, infrared and visible lasers can mess up their vision systems etc.

Those things run on batteries, which have a pretty short lifespan. Also, there are plenty of patriots that know how to program robots and would happy to take them over.