Firstly, to hire women at junior levels and invest in them only to lose them before they can take on the senior roles is a poor outcome business-wise, returns-wise and image-wise.
Secondly, women are increasingly becoming the decision-making consumer. Not having a proper representation of women at senior levels would mean firms are losing out on an opportunity to have leaders who have a better understanding of the needs and psyche of their target consumers. That is a bad business decision.
Thirdly, women will increasingly be the decision-maker and enterprise-buyer on the corporate side as well. Not having top leaders who can easily relate to the situation of senior female executives across the table can have adverse consequences.