Losing weight isn't difficult to understand, and knowing is half the battle. I'll explain:
They eat too much sugar because sugar is very nearly in every cheap processed food. It works like this: high blood sugar + insulin = instant fat storage. So basically not eating well is the same as eating too much or too little of the things your body needs. We feel hungry when our body needs macronutrients (proteins, carbs, fat) and micronutrients (vitamins, iron, zinc). If the body doesn't get the nutrients it is craving, then it will feel hungry forever. And when we feel hungry, our body thinks it needs to conserve energy, so it makes the person lethargic. So starts the vicious cycle.
Another fun fact about sugar: Cocaine and Heroin both less addictive than sugar. And the fact that the food industry is so rich they have had nutrition labs synthesize them chemicals that make food even more addictive and seem better than it really is. MSG is a classic example of this. The trufax are that sugar is a dose-dependent heptotoxin that slowly eats away at your liver's ability to function over years of having high blood sugar, eventually developing into diabetes II. Not only that, but sugar molecules have sharp edges, and excess sugar in the bloodstream creates millions of tiny lacerations, which if not allowed to heal can destroy the vein and decrease the circulation to that part of the body. This is actually the mechanism that prevents diabetics from healing like the rest of us, and eventually leads to things like your fingers and toes falling off from a minor cut because they failed to take care of themselves.
They are addicted to food as much as a smoker is addicted to tobacco, the main difference being we actually need to eat to stay alive. The dumb irony is that eating right and not exercising is more effective than eating poorly and exercising for losing weight. They keep their old shitty eating habits and then face frustration when it inevitably fails them. Ever wonder what they put in to keep the flavor in "fat-free" labeled products? There are several dozen sugars, your body treats them all the same way.