For girls who wear sweatpants everywhere: why?

What kind of sweatpants are you talking about? Tight yoga-type pants or the baggy fleece kinds? If you're talking about tight ones (which I ume you are, because lately, almost every girl is seen wearing tight TNA pants, but not so much the baggy ones), then that's because guys find it attractive. Guys think that a girl's looks hot in those kinds of pants. So that would be to attract guys. But for the baggy ones, that's usually just comfort.