Do you know the difference between education and experience? Education is when you read the fine print; experience is what you get when you don't.
Although there is no clear answer, I would say that it is because women are considered more of a liability. Women have can get pregnant, and have to go on a maternity leave, and are still considered to be not as tough as a man. –Meaning they can handle less labor intensity and hours. This is actually a very interesting topic. If you care to learn more about it research “gender wage gap” and “Equal Pay Act.” -Some believe that gap is diminishing, and some believe it is widening. You be the judge.