wowzz
Because airlines sell seats to passengers (including a hand and checked luggage allowance.)
As I understand it and in simple terms, the airline prices the flight by looking at the aircraft volume and weight/balance constraints and then pricing seats and air freight to maximise the return (yield.) For the sake of argument, let's disregard yield management techniques.
The average pax weight is assumed and relies on a distribution across a range, so the individual weight of a passenger is immaterial to costing a ticket.
Airlines tend to sell passengers space at a premium, rather than weight.
Currently, your question doesn't fit the model, it would be the equivalent of asking a private school why the slowest kid in the class doesn't pay higher fees than the brightest, as s/he takes more of the teacher's attention.
Maybe the model will change in the future.