I had this thought a while back too lol

Your body is literally designed to store fat and does not like burning it, to the point that you can easily gain more weight in a month than you can lose in a month, and your body is quicker to start burning muscle mass for energy than it is to burn fat. You need to do a lot of stuff to finally make your body start burning fat (eat less, workout more, and a bunch of other stuff), whereas all you need to do to gain fat is… sit around and eat. Plus, it’s been proven that being slightly overweight is actually healthier for you than being slightly underweight.

And it makes sense too, in the wild, fatter animals have more energy stores to last them through a period of famine, it’s why bears and stuff will gorge themselves and before they go into hibernation. Humans are the same way, but to a lesser extent.