Anime clothing obey the rule of cool, not physics.
From tvtropes, "impossibly cool clothes":
Animated characters can often be found wearing clothing that just looks too good to be real. This can be proven by the occasional attempt made to reproduce these clothes by the costume teams for live action adaptations or by enthusiastic fans. Sometimes this is because of structural elements that flout or ignore real-world physical laws, or because the outfit in question requires an inhumanly perfect (or simply inhuman) body underneath it, or because real-world materials just don't look as good as ink and paint.
Just go watch cosplayers cosplaying a slice of life anime like Lucky Star or Azumanga Daioh. Their costumes look unnatural compared to an actual photo of a japanese student. They don't look like students. They look like adults in gaudy costumes, even the pretty/good cosplayers, even those who are the actual appropriate age.
A mangaka only has to design clothes that would look good on paper. This is no guarantee it will look good in real life, and, if anything, cosplaying has proven that they don't. They won't look fashionable; they'll stick out like a sore thumb.