In the age we currently live in where food is plentiful for many people (though certainly not all), it probably seems like 2000 calories isn't all that much to eat. The reason we need about that much food to survive is that our ancestors who required no more than that were the ones who could survive on what food they scrounged, acquired, or found. If they had not started to store fat when they consumed more than about 2000 calories, they would have died out.
I learned awhile ago that Cro Magnon, who were a different branch of human, had much bigger bodies and needed 3000 calories to survive. That means they had to spend significantly more time finding food and had to find more of it. That being the case, they probably didn't have the time to develop culture and tools because they spent so much time finding food.
At any rate, when I learned that Cro Magnon could eat up to 3000 calories per day without gaining weight, I felt a twinge of regret that we didn't develop with big bony foreheads and barrel-chested upper bodies. It's quite a testimonial to my food addiction that such a thought would even occur to me. Nonetheless, we'd all be a lot thinner if we had descended from Cro Magnon, though we certainly would be uglier and dumber.