Plane ticket prices

Why is it the norm for plane tickets to be the same price, regardless of how much weight they account for? In other areas of logistics it's understood that customers will have to pay a higher price the more they need to transport. For example, there's no expectation that I'd pay the same price to ship a 50 pound and 100 pound box.

Given that passengers' combined body weight and luggage weight
often varies by at least that much, I'm surprised airlines haven't introduced more nuanced pricing. They've already taken the unpopular move of charging for domestic check-in luggage and food service. It seems like they should take this to the logical conclusion and charge for the costs they're incurring, namely fuel. Importantly, this isn't just about body weight; it's about the total weight that people are asking to transport.