Because your minimum is not more or less valid than a minimum demanding full length clothes. Or some rule that is targeting outfits specifically identified with black hip hop culture, like it is quite popular among american schools. And demanding that no underwear is visible in a classroom is weird in so far, that this seems not to be an issue in general, because that is already a thought of students and their parents, without any formal rule. So the example, while culturally and morally totally understandable, i also agree with the statement of you, is not so much relevant to the situation in schools. And there these rules are arbitrary.
Just to give on example, one could also argue for hairstyles, that they shouldn’t be so long, that it represents a tripping hazard. Noone sane would disagree with it, but it doesnt need to be formalized in an explicit rule, because it is obvious and not a relevant issue.