Because they’re the people who’ve spent their entire lives refusing to accept satiation. It’s no different from asking why people who weigh 400 lbs are so hungry all the time, after all surely they have more than enough fat reserves to see them through all but the most apocalyptic famines. But no, you don’t weigh that much without having completely destroyed your sense of satiation, and to maintain your current state you need to continue overconsuming. Furthermore to begin to scale back even a little is uncomfortable.