Why do I hardly ever see women at the gym?

Do I go at the wrong time? For years, I've been going to big gyms such as Gold's Gym, World Gym, Bally total fitness and hotel gyms when I am out of town. I work out daily, usually anytime from 6-10pm. I've noticed that most of the people in the gyms are men. This includes the people weight lifting as well as the poeple on treadmills and other machines. The only time I ever really see a large group of women in the gym, is when there is some kind of a group aerobics class going on. Why is this?