@Mah_Buddy_Keith For no reason other than that the school says so (I mean, there is probably some justification like avoiding accidents, not bothering the neighbors or not harming the school's image, but that's the basic reason). Private schools often have seemingly non-sensical rules like those which you are expected to follow, including stuff like wearing your uniform in a particular way, not having "unnatural" hairstyles, etc.
It happens in western schools but it seems to be more widespread in Japan, according to the extremely reliable source that is the anime and manga I have read instead of doing any sort of research.