As a woman, I'll tell you why this is the case, and many people probably won't like my answer.
Patriarchy. For millennia, women have been subjected to being second class things-- objects of desire for men (particularly ones with wealth and/or power), so it's like second nature to objectify the female protagonists in our entertainment, and it becomes necessary to do so, just to compete within the Establishment (again, the ones with wealth dictate the rules we all play by).
Luckily, with all the chaos that's unfolding (at least in the West) recently, I feel that there is a monumental shift occurring culturally that's going to finally place women on equal grounds with men, dissolving the ongoing cancer of Patriarchy. At least, that's my hope. We're all human beings. We all have red blood coursing through our veins. We all require clean water to drink, clean air to breathe, and nutritious food to power these biological mecha we inhabit.
(And as a footnote here, please note that I am crazy-- but in a good way.)