Well, that's sort of poorly thought out...'
What if all the movies nominated only had supporting roles that were male? You know, like Shawshank Redemption, Hunt for Red October, Saving Private Ryan.... then women would be furious, storming the gates, accusing the Academy of gender discrimination.
It's so much better to have different categories....
Besides, in case nobody told you...... men and women are different. They cannot do all things equally well, no matter what the "experts" tell you. Both genders have their strong and weak areas. It's how God made us.