There are many other nations that reached comparable prosperity and food abundance at similar times, yet don't have even comparable obesity rates. An extreme case is Japan, at 3.5% obesity rate maintained largely though willpower and cultural norms, by literally firing those that are too fat, or in any case applying strong social pressure and mandatory counseling.
1. There are other differences in Japan -- most obviously a very different diet
2. Obesity rates in other countries are catching up to the United States.
3. The first two points call into question your claim that Americans are uniquely lacking in willpower.
4. I don't want to live in a society where people are fired or ostracized for being fat because it is cruel and inhumane.
5. The US has a $66 billion weight loss market, suggesting many Americans do want to lose weight (cf., https://www.prnewswire.com/news-releases/us-weight-loss-mark...), but I'll leave it to the oracles to decipher what you mean by "really" want.
By this metric some three-quarters of Americans have poor impulse control (cf. https://en.wikipedia.org/wiki/Obesity_in_the_United_States#/...), so I think it is actually the other group that is exceptional.