We're charged for extra luggage, because there are limits to how much weight an airplane can safely carry. But, of course, one of the things that passengers lug onto planes is themselves. An underweight person with extra luggage may well contribute less total burden than an overweight person without. Doesn't it seem strange to charge the less burdensome passenger, in such a case?
It would seem more sensible for airlines to calculate the total per-passenger weight allotment, and charge those who exceed it. (It makes no difference to the airplane whether what's weighing it down is in a suitcase or a suit and tie.) In the present system, we effectively have thin passengers subsidizing the overweight. Is there any good reason for this?