Nuclear weapons are perfectly capable of doing exactly the same thing, but that hasn't happened yet. Why? Because no one wants the same thing done to them in retaliation. Droid armies wouldn't be used on civilians due to the fear that the other side may use it on theirs.

You inevitably run into the reality that doing so would be mutually assured destruction by the promise that the opposing side could then do the same to your people. The idea that we're going to keep sending soldiers to fight wars when robots become vastly superior is a losing proposition by all accounts of game theory. And, in all likelihood, when this is achieved, we will see treaties emerge that forbid use on civilian populations just as we've seen for dozens of other weapons in history. Of course, terrorist states will always break these rules, but in the end, the cumulative number of lives saved would vastly outweigh those lost.

If everyone fought entirely with droids, no one would need to die in future wars. They would be fought entirely with money and manufacturing.