Well, yes, but the point is, ethics aside, if it was as simple as just hiring from an entire subset of potential employees who are cheaper than others, why wouldn't you do it in order to have a more profitable business?
You wouldn't do it because it would be unethical. That's like saying "besides the fact that eating a cheese burger is terrible for you, why wouldn't you eat a cheese burger?"
Happy? Besides, you've made quite a baseless assertion when assuming that there exist employers willing to exploit the fact that women are paid 18% less.
Let's try again. The argument here is, basically: if it's true that men and women are interchangeable as resources of labor, then the wage gap can be seen as an opportunity for arbitrage. Any capitalist would just hire women (because the information the market has is based on the assumption that a woman can provide the same work as a man for a lesser cost), market forces would bring women's pay up, the wage gap would diminish until we reached an equilibrium.
The argument is far from perfect (it ignores social and cultural aspects), but it is pretty reasonable as a way to see that if there is a difference in pay, there is a difference in aggregate output. What are the causes of this difference in output is subject to another topic, one I can't say I have an explanation or consolidated opinion.
About the ethics of it: there is nothing unethical about choosing who to hire based on cost, if the quality of the final product is the same (and if the contracted parties are not doing anything unethical themselves). Let's remove the sex factor out of the equation... if a woman living in NY asks for $100k for some work, and another woman with "comparable education and experience" living in Poland asks for $30k, is it unethical to hire the Polish one?