Pay by weight rates for air travel

Just a thought, but why don't we have pay by weight rates for air travel? The plane literally uses more fuel to carry more weight, and the actual flying weight has to be calculated relatively accurately to prevent the plane from needlessly (wastefully) having to carry more fuel than is needed (within safety margins, of course). Rather than a standard fare for passengers weighing 15kg or 150kg, wouldn't it be smarter and fairer to pay by weight? I see how the overweight majority would complain about this, but it isn't unfair treatment. It's fair treatment. For example, on some major airlines nowadays, a small person weighing 50 kg gets one 20kg bag of luggage. This person accounts for 70kg of the plane's load. The next passenger weighs 120kg and gets the same 20kg bag allowance (a total of 140 kg, or double the first load) for exactly the same price, even though it costs the airline more in fuel to actually move that load. Then there's the idiocy of hand luggage. They can both bring the same amount there too. And if, for example, the small person exceeds the hand luggage amount by a few kilos, they have to pay extra to check the extra weight, usually at insane prices. Yet logically, in terms of actual costs, they should still be paying less for a total weight still only about half of the large person's. 

I don't think the fare should be entirely weight-based. But a fair price would have a seat-based fee (incorporating the cost of flying the plane itself) and a weight fee on top of that. And then you should be able to decide when booking the ticket how much weight you need. And that should be that. I pay for my seat and then can add on as much luggage as I need, taking into account my own body weight.  So if the small person buys a ticket for 120kg, they should be allowed 70kg of baggage for the same price as their large friend travelling with nothing. (This example is getting out of hand, but the principle stands.) If I travel with nothing, I should pay less. If my child accompanies me, he should pay less still. 

I think airlines are sort of moving in this direction, but from political correctness haven't quite taken this step yet, though baggage is counted separately and some airlines still have children's rates, for example. I think it should be taken this extra step. Have to include your body weight at least when buying a ticket. Rounded to the 5 kg or so. 

But fairest would be to have the person, bags and all, up on the baggage scale to pay the actual rate at the time of departure. And pay extra for overweight above that booked. Not above some stupid arbitrary limit. 

Just a thought.