I once worked with a champion saddlebred who would try to flip over backwards if you tried to hold the curb rein from the ground. This particular horse in the little video was given to me as star grazer who went from being completely hand ridden in a twisted wire snaffle to a western grazing bit with no contact. It has been slow work to get him to trust the hand. Sometimes taking many steps backwards.
I am not saying your are wrong when you ask and I'm paraphrasing, "why not from the ground? the horse should respond " and I'm sure he would probably be accepting of that work now, but one has to adapt to the horse's baggage and personality not just go by the book. I am willing to listen to the horse and compromise where necessary if it means getting results. I am not a professional on a time limit. I'm a 60+ pleasure rider having a ball.