Why do kids not like to zip their jackets?

I am curious to know that because my LO always comes out of school with her jacket wide open so, if its unpleasant weather, I always ask her to zip her jacket. I am not the only parent at the school gate that is heard saying the phrase "it's not nice weather outside, so zip up your jacket please". 

So that is my question, why do children need asked to zip their jackets and, is that not something the teacher could ask them to do before the child gets outside?