Because once the modern food addiction is broken dieting/fasting is relatively easy and painless. People like this go from withdrawal symptoms if they don't snack on modern foods every hour or so, to not even thinking about it once the addiction is gone. While fasting blood sugar is completely stable, the brain's apostat resets itself and thus the subject loses cravings for modern foods. The average person (not even fat) will usually snack at least every 3 hours or so. That's how dysfunctional people are but it's so common that it has been normalised.