[Boy ain't that the question.]

Humans live below the ground. We only eliminate the ones who come up above the surface. As to why...

[He looks out the nearest window, nodding at the trees.]

Even with all the travelling I've done, I've never seen a forest so lush as that that spanned more than a few miles. Most of our world is dead desert. Humans did that, before they were forced underground.

[Or: canon doesn't actually tell us, but I can bullshit a reasonable enough story about it.]