>Because for like 99.9999% of evolutionary history, calories were scarce and unpredictable. You were WAY more likely to die of starvation (due to sickness, drought, winter, injury) than somehow finding so many calories that you got fat. So we all evolved the strategy: "when you do find a high-cal food source, eat as much as possible!!" And in those calorie-scarce times, (aka essentially all of human history!) that was a great strategy to have. It kept your ancestors alive.

Also we have the "turn those excess calories into fat, you might need it later!" strategy - hence we get fat.

Because for 100s of thousands of years our ancestors died of all kinds of other causes before they could get fat in addition to having a calorie deficient diet.