If men get paid more than women for doing the same job, why don't employers employ only women?

To a certain extent, that's what happening. Didn't you hear the term "mancession", used to describe the large number of men laid off during our current recession. Obviously, they can't replace men entirely. There are certain jobs that are too physically taxing for women to perform as well as men and it would cost employers too much. But you can bet that corporate employers will exploit any worker they can.