The human body needs fat and muscle to be in tandem. Deviation to the extreme of either end is very unhealthy and is quickly corrected by the body, given bare minimum stimulus. Absent this, you'll get the beetus and other things. High body fat can also protect you, for example, obese smokers are healthier on average than skinny smokers. All that poison goes to their fat, because fat pretty much acts as a sponge in the body. That's why "pink cheeks", "baby cheeks" are a sign of long-term health. It takes a very long time (years, if not over a decade) of a proper, modern-toxin-free diet to remove toxins from the body. In the absence of proper amounts of protein and fat your body usually likes to isolate harmful things, which can lead to myriad conditions down the line, even if you've been exercising for years and have been "healthy".
Also, related to the title of the main post, sugar and whole grains (processed flour is basically sugar) and horribly addictive. Obese people that have difficulty quitting / have side-effects from quitting might as well have entire regions of their brain dedicated to timing the intake of sugar precisely, at the expense of their mental functioning. So they would inevitably feel out of energy / motivation after a while, and would need food.
Also, the easiest way to lose fat and gain muscle is to subject your body to external weights. It doesn't even have to be very heavy. If you're obese, you could OHP 10kg every other day and lose a significant amount of fat, even without changing your diet (assuming it's not really horrible)