Given the success (inasmuch as one can call it that) they've had with drones, why bother with mounting a gun on a robot that can barely run for longer than an h

Given the success (inasmuch as one can call it that) they've had with drones, why bother with mounting a gun on a robot that can barely run for longer than an hour? Wouldn't a remotely-piloted/autonomous vehicle with guns mounted on it be much more efficient than a robot walking around on 2 legs? I guess I just struggle with understanding how this changes anything.