Why do so many girls wear cardigans and extra layers in the summer?

maybe i am just warmer than other people, or get hotter faster, but i dont understand how some girls can wear jackets/cardigans in the summer time! its so hot XP and i see soooo many girls sporting 2-3 layers too! or, oh my goodness, skinny jeans! and skinny jeans are warm in general! why do they do this? just for fashion?