You're taking your first world, society of plenty as the parameter, when it isn't. As near as the 19th century, before the advances in food production, food consumption accounted for 80% or more of one's salary, and that's considering a very advanced civilization. Imagine what a not-so-advanced one would be like? Or, even worse, pre-agricultural ones? Traditional food production (not to mention hunting-gathering) techniques are extremely ineffective.
Thus anything that provided for less energy consumption, including the fact your muscles deflate very fast once you stop doing exercises (strong muscles consume more energy), and that eating even small amounts can build up to obesity, comes down to an actual, evolutionarily-sound reason of saving as much energy as possible. Ancient environments are quite energy poor, and nature selected for that.