Like it or not, the self proclaimed overlords are making sure robots advance as fast as possible.
Got to get a thing to control the masses.

However, every time an advancement in robotics gets created, it makes defense against robots easier and cheaper.

So, you got a million dollar drone. It flies into enemy territory and delivers its destruction.
However, for about $10 each, you can build a flying brick. A drone who's only program is to get in the way of things flying in.

And things flying, meeting a brick, do not fair well.

Similarly gun emplacement robots are far cheaper and far superior than any of the moving piles of soon to be junk.

An attack platform has to be all terrain. Has to be able to work its way to the target.
Has to have fuel/energy to get there.
Has to carry all the munitions with it over that distance.
Has to be able to recognize enemy targets that are constantly changing.

But, lets say you made a automated gun turret.
It is really simple. The robotics are beyond easy.
It can be programmed to recognize each person you want to keep safe.
It can be programmed to know the lay of the land.

So, in any economically near balanced battle, attacking is going to become prohibitively expensive.