Why? Because dumb women - and most women in general - can't follow logic, that's why.
A sensible woman knows when she's getting too big, as that "stop sign" comes when she's 5lbs to 10lbs above her ideal, so they stop there and then and don't gain more weight. These dumb landwhales let themselves just get bigger and bigger, like a snowball rolling down a hill gaining girth, and by the time they do want to lose weight, its gonna take momentous effort to lose it, like your talking 1 year easily of a military diet and strenuous 3 to 4 -day a week exercise to lose 70 - 100lbs. 9 months for those in the 50-65lbs overweight category, and 7-8 months for those in the 30-40lbs category. Most whales don't have the drive to do this, cuz the exercise required is too exhausting and the food is too bland. But mainly because there's always a normal-looking simp willing to fuck them at any large weight. If ZERO men fucked fat women, they would HAVE to lose weight.
Obese men without cash/status KNOW they have to lose weight to stand any chance of getting in bed with a passable-looking women.