It is a biological defense against starvation...developed by humans over tens of thousands of years of adapting to poor hunting seasons and bad harvests. It's why you're cold all the time, why you get constipated, why you run out of go, why you can feel cloudy and not quite yourself....your body just shuts down everthing possible to keep you from losing anymore weight because it thinks you're dying of starvation. Lots of critters have the adaptation. Torpor. Brumation. Hibernation.
Uban legend has it that sometimes kicking up your calories a little bit and doing more physical activity, getting more sunshine...can help snap your body out of it.
In another ten thousand years, maybe our bodies will develop a resistance to the excess calories that are shortening our lifespans.......but for now, they're still more into defending against starvation. If only we could explain it to them. LOL.