The evolution of mankind has been rocky at best, and food security has always been a struggle (and for many still is). Mark Mattson, head of the National Institute on Aging's neuroscience laboratory, believes that, "evolutionary pressures selected for genes that strengthened brain areas involved in learning and memory, which increased the odds of finding food and surviving.”2
In other words: you had to be smart to survive, and in past times that often meant thinking on an empty stomach.
Our evolution through times of famine and scarcity has taught our body to function at its highest levels in the absence of food. Even at the most basic level, the way our body uses and stores food is built around the idea of surviving and thriving without food.
That spare tire you hate? Your body spent millennia developing the ability to create that. It ensures that if you had to survive a harsh winter, your body would have its own internal fuel source.