I dunno. I guess because they can. But as a woman I can say that any woman who wears one and wonders if her butt looks big should be institutionalized.
Believe it or not, there's an entire marketing niche for women that size. You'll find some terrifying things if you google "Abundant-bodied women."
(Before you ask, I was looking for an exercise outfit when I was 7 months pregnant.)
agreed!