I don't think anyone knows for sure, but I think the prevailing theory is it being a survival mechanism.

When our ancestors faced famine, it makes sense for the body to shed as much muscle as possible, since this reduces the metabolic rate in the medium-long term.

Muscle is more metabolically active than fat. Although fat can be used up for energy more readily, but muscle takes more energy to maintain. Burning fat just to maintain (unnecessary) muscle doesn't make sense in terms of survival.

Could just be its for winter where you don't need to move much for a few months, otherwise normally you need that muscle to gather food even when starving, someone has to gather it and it wont be someone who shed most of their muscle.