i don't think it's a question of social norms. if people were adhering to pre-mcdonalds social norms, they wouldn't be going somewhere besides their house (a restaurant, but one without waiters) and sitting on brightly colored plastic benches eating mechanically separated chicken glued together with modified food starch cut with texturized soy protein

people's appetite is evidently for calories, and they commonly regulate their caloric intake or metabolic rate to within 0.5% in order to maintain their level of obesity. social norms are not capable of that kind of precision

consider, if you gain only 5 kg in ten years, which many obese mcdonalds eaters in the usa do, that's a caloric excess of 5 kcal per day. half a gram of fat per day. that's 0.3% of a metabolic rate of 1666 kcal per day or 0.2% of 2500 kcal/day. if they were to eat 0.3% less calories than they were burning instead of 0.3% more, they'd be losing 5 kg every ten years, not gaining it

so the homeostatic feedback mechanism is almost working. it's clearly not completely broken. its error is well under 1%. but it does evidently have a systematic error across the population. and fast-food optimization for profit by itself is an insufficient explanation for that