Obesity isn't really as avoidable as a checked-in bag that many, with some responsibilty, could do without.
How many people flying with Ryanair or EasyJet for instance will take a 20kg each? Probably not a lot! We take a max of 2 20kg and at a push, 3 15kg bags between 4 people (2 male, 2 female) for 2 weeks away because, like may people, we won't pay extra for luggage we realistically can manage without.
Given the chance to have a 22kg bag each for no extra cost, like near enough every person on the planet, we'd probably take advantage of the luxury or even feel obliged to get the value out of the total cost. But we can easily manage with much less and the charges make us more repsonsible, and if everybody could do that, think how much uneccessary weight, time and money they'd save!