“If women get paid less than men why don't employers hire only women?”

Assuming men did make more (they don't), here are some reasons you might prefer, or have to hire a man: Men do tend to work longer hours. They also tend to be willing to take on greater risk. There are some fields that are >99.9% men (Ex: garbage men). That type of sexual discrimination in hiring might be illegal.

However: Women don't actually make less than men. Old men make more than old women. Young women make more than young men. Last I looked the break even point was 35. Both times I heard about a salary adjustment with an aim to combat discrimination it benefited men more than women. That might be an availability bias though.