Why do so many US Political arguments involve attacking the extremist positions of each side?

Its a version of the Strawman argument. Define your opponent’s position in the worst possible light, frame it as such and then attack the frame. The frame will crack creating the false illusion that you have discredited the opposing side. Its a cheap trick and is extremely dishonest (not to mention illogical). Why is it used? Because in the eyes of a partisan audience it appears as though you have pulled one over on your opponent.