they need a mixture of 3-4 seat widths, scattered randomly, with wider ones costing a little more, since wider people = more weight = more cost to fly. I see no real reason why weight is not a factor in costs. Children often fly for less.
Assuming a delta of 50kg that's $5 between a light person and a heavy person, which is far less than the price of the ticket.
The enforcement scheme (setting up scales, getting complaints about being weighed, etc) has its own cost.
That's not to say that weight isn't a factor. Samoa Air charges by weight: https://www.theguardian.com/world/2013/apr/02/samoa-air-pay-... . I believe there is a wider range of weight in Samoa and the fuel costs are higher.
I've read a number of articles that conclude airline seat sizing is deliberately designed to be just at the point of unbearable, to persuade you to upgrade to the next class. Frustratingly, the next class up costs at least twice as much. You either have to pony up a huge sum for semi-comfortable seating, or try and bear it. I've flown London-Hong Kong (12 hours) in Cattle Class a few times, and that is about the most I can stand.
What’s next ? Charging disabled people more because they have higher handling fees for the airliners?
We live in a society and we should set some standards that accommodate everyone without discriminating else we are lost!