Let's just ignore your misanthropic, fat-shaming trolling here and pretend you asked an honest question.Why do we even research fusion ... this is clearly the future of energy.
Why don't loose people weight, despite the fact they take in far less calories than before.
There are mainly two reasons.
Firstly, digestion becomes a lot more efficient. People ware fixated on calories as the amount of energy that food contains, but calories are a crutch. They were originally calculated by burning food in an actual oven, nowadays they a calculated. The problem with this is it is not accurate, because we humans don't have an oven inside use. Digestion is a chemical process and it's different because of genetics, hormones and gut bacteria.
A normal, healthy person does not convert all the food they eat to energy. Their body does not need to. But if need arises, if you are starving or dieting, the digestive system can extract more energy from the food, e.g. by producing more stomach acid and slower bowel movement. But the most important thing is gut bacteria. In a pathologically obese person or someone who had a stool donation from a obese person, the bad gut bacteria will digest normally non-digestible food like fibers and the sugars and fatty acids produced by this go directly into the blood through the bowel mucosa. The less you eat the more efficient the digestive system becomes over time. And it stays that way. So less amount of food can produce more energy. That's why dieting always never works.
Secondly, if need be the body can go into energy conservation mode. To do so, it lowers the body temperature a bit and the temperature of the extremities. When you have enough bad gut bacteria, the body will now produce fat cells in the skin with the saved energy (the link is unclear here, but it is believed the bad gut bacteria can produce hormone-like substances that trigger this systems, if you have enough bad bacteria). The fat works as an insulator, so obese people don't need as much energy to maintain their body temperature as a result they don't need as much food as before. Also obese people often feel tired and energy-less so they avoid energy-intensive activities. It's a vicious circle.