Maybe poor food quality does contribute, but I don't think it is the main factor.It's a mix of socioeconomic factors. Cheap, processed junk that is utterly overloaded with sugar is both very cheap and convenient, which coupled with long working and commute hours makes it the more attractive option for the overworked compared to cooking with their own fresh ingredients. Compounding on this are the portion sizes served in the USA; compare and contrast to those of other developed nations and you will notice just how much larger American servings are.
Anyone with truly prodigious BMI almost always have a host of different psychiatric diagnoses. Because generally people who value self care and love themselves would not allow their body to become like that.
To be clear, I'm talking the level where fat is impeding your daily life, making it hard to do basic tasks like wiping your ass or even walking around. Not having a some flab, a beer belly or lack of muscle toned stomach.
I think in Chinese culture, mental illness is simply not expressed in the same ways as in American culture. They have different coping mechanisms. (not necessarily better, just different)
Strict anecdotal disclaimer, but the Chinese people I've seen that get people of Walmart levels of fat almost always had mental handicaps that they are unable to judge that they're eating too much, essentially having the mental capacity of small children.
In contrast, what you seem to see among many Americans are able bodied people but with food addiction.