If every seat number had bin space allocated for it, that space would not be large enough to fit even the smallest carryon suitcase. Overhead space only works b

If every seat number had bin space allocated for it, that space would not be large enough to fit even the smallest carryon suitcase. Overhead space only works because not every seat uses a suitcase's worth of space (and stuff smaller than a suitcase usually gets crammed under the seat in front of you).