Because our brains are the most developed out of all of the other species on this planet. Also, we require the most maintenance to survive. We can’t live in the ocean, such as a fish, shark, dolphin, etc. but we also can’t live in the forest such as bears, deer, coyotes, etc. we need to live in a stable environment when we will have enough food to survive as well to live in certain temperatures that are best suited for our kind. Hopefully that helped a little bit