Because the "pay gap" comes from the fact that men and women in aggregate make different choices with respect to how they develop their human capital. When you

Because the "pay gap" comes from the fact that men and women in aggregate make different choices with respect to how they develop their human capital. When you compare groups of men and women who make the same choices with regard to their careers and what to study, the gap disappears. In the case about 20% of college graduates who studied Computer Science are female, and about the same proportion are seen employed as Software Engineers in the industry. Those women are not any cheaper than their male counterparts, because they're just as qualified. Why women and men make different choices in aggregate, as well as if any of that is problematic, are topics of further debate that I don't think we have concrete answers to yet.