Sam, you didn’t answer the question. Namely, how is a frum woman wearing pants designed for women more of a tznius problem than a woman driving?
Also note that if a woman drives, and has a car at her disposal, she will naturally be “out and about” traveling more frequently than if her travel were limited to traveling with her husband or public/paid transportation. And you then run into more instances of unnecessary travel that the Shulchan Aruch says not to do.