Why are leggings suddenly a pants substitute?

I generally have nothing against leggings, I own a few pairs to wear underneath shorter dresses or skirts, but lately girls have been wearing them to school with nothing over them. They just wear, for example: Black leggings, Ugg boots, and a sweatshirt. This same old same old style is extremely common at my school, and honestly I think the leggings with nothing over them look is trashy. Its just girls trying to show off their es, right? Leggings are tight, and with some girls I can see the outlines of their parts, wtf. When did this become okay? And leggings wearers, why do you do it? Be honest, just showing off your es? :P