Why do you believe that? What if it is the case that women prefer less stress and more work/life balance in their careers compared to men, and thus fewer of them, proportionally, choose to make the sacrifices that it takes to occupy the top positions?
Heh. Women who choose to take those 'less demanding' positions to have a 'better work/life balance' are generally balancing raising families and rearing children in addition to their paid jobs. If you think this is choosing a 'less stressful option' than men working at those high paying stressful jobs, clearly you have never been a woman.
I used to know a male oncologist whose kid attended the same daycare as mine. The day care required that all parents spend X number of hours a quarter in the center, working with the kids. He started sending his wife instead as he found half a day with preschoolers to be much more stressful than his work with cancer patients.
When I went back to work after my first child was born, I could hardly believe how much easier my job was compared with being home with a baby. At work, I got potty breaks! And lunch! And adult conversation--or something close to it.