They aren't all ladies, providing Banshees are the ones in discussion. As explained in Path of the Warrior, it's just that female Eldar are more susceptible to that line of emotion.Let's just chunk down to the Eldar. The Eldar have a fully integrated military - including an all-female unit, which in my opinion passes the "interesting reason for stupid gendered stuff" test, because it's entirely arbitrary and for no earthly reason at all - but we only have models for female footsoldiers and the all-lady unit