Why did the Evil Patriarchy, along with other systems throughout the world, exist? Some might say that it was all about men exploiting their larger physical stature and aggressiveness to subjugate women. Probably a large number of people would say that. The narrative said as much, so that was good enough for them. Of course, narratives have been wrong. Such is the case here.
People appear to have predisposition toward certain classes of behaviors. Probably all of them, at one time or another, have proven to be useful for the individual. Some have even proven to be useful for the tribe. The purpose of the Evil Patriarchy was attempt to encourage pre-disposed behaviors that benefit the tribe, through providing two effective attentive parents for the children. Naturally, well-adjusted children make for good tribe members in the future, perpetuating the tribe.
End of story.