[atoav]

Because most people weren't doing hunting or labor because they liked it[1]. They just liked dying even less. People of generations before modern times led often extremely short and miserable lifes. Not moving meant dying. Moving more than was necessary also meant dying.

We werent designed to do sports, it is just that those who didn't move were a little more likely to die. And people who are more likely to die are less likely to pass their genes on, simply because they had less time to reproduce. The result: We have bodies which are meant to be moved, but only if needed, like many other animals. Moving too much was bad too, as resources are needed to do it an resources were not readily available at all times [cue picture of rough European winter ca 1300 AD]. You probably guessed already that starvation, because you burned too many calories has luckily become a non-issue in most parts of the world.

So we are a direct result of the environments and circumstances our ancestors lived in, managed to get kids in, and who in turn managed to survive long enough to raise their kids.

[1] Many people actually like moving or grow to like it. A bit like with spicy food, which hurts at first, but pours out endorphines, being fit and using your body can give you all kinds of good emotions during and after the activity and you can grow to like it once the pain is over. Being able to rely on your body and know it and its limitations can be very fulfilling. When I run my Sunday 16km I don't feel any pain. But I am fit. I might feel pain when I attempt a marathon and try to set a personal record. But regular running is a breeze once you are in shape.