This is ridiculously naïve. You seem to be completely forgetting the existence of addiction.
> One example is how easy it is for 99% of women to quit social drinking when they get pregnant.
Because they weren't addicted to drinking, like most healthy people.
Do you think a single fat person wants to be fat? It's uncomfortable, inconvenient and embarrassing; nobody wants it, and they know exactly what causes it. The reason more than half of our population is overweight is they are addicted to food and our society not only enables it but encourages and reinforces it. They can't "just stop".