Because although the % of gameplayers are relatively even, the type of games they play are not.
Okay, let's try this approach.
Demographic A has been viewed as the primary market for an industry since its inception. The industry designs and builds products specifically to appeal to their interests and tastes. This group has been heavily catered to, marketed to, and made to feel as if the industry exists to serve them. Use of the products becomes a shared hobby among many members of this demographic.
Demographic B is ignored by this industry, and in many cases, made to feel specifically excluded. As the industry grows and expands and starts building other types of products, this demographic actively engages those products in large numbers. But the barrier between Demographic B and the products heavily marketed to Demographic A is not breached because the image and marketing approach for them does not change.
Yes, I really do believe that more women (and seniors) would be enjoying a lot more different kinds of games if the image created by marketing were more welcoming.