The styles are part of it. Other factors would be...

...the need to wear pantyhose: they give a smoother line & help cover up yuckies like spider veins, but hold in moisture & are killer in the summer heat.

...what's in style: the fashion industry has pushed slacks more than dresses & skirts, at least in the teen world. Those of us older than that tend to wear what we want.

...and...Fat Thighs! They rub together most uncomfortably. Tr*****s put some fabric there that prevents this.

I hope that's not TMI, but sometimes Truth is painful.