My suspicion as someone with lifelong weight struggles and having tried GLP-1 medication: overweight people require more willpower to lose or maintain weight relative to those of normal weight.

So the advise or admonishment of the normally weighted that losing weight "just requires willpower" is true but facile.

If we were to medically induce a constant feeling of hunger and insatiability into a person of normal weight, I'm sure they could keep the weight off, but would find that their willpower is highly depleted.

There are medications that cause increased appetite and weight gain (ex: some bipolar depression medications, prednisolone). This effect is so pronounced, that if a doctor sees the patient not gaining weight, they will suspect non-compliance and have to rule it out. Of course, some patients use extreme diet and exercise (willpower) to avoid these effects, but a normal person accustomed to expending a normal amount of willpower to maintain weight will find themselves gaining.

Over-eating is not strictly a choice. Corporations spend billions on manipulating the public because it works. Regulation is needed, not willpower.