Let's see...
In the wild, animals have to search for food and have to fight for survival.
In a zoo, they stay in their cages, a safe distance from humans with a wall to block them, separating the humans and animals and making sure they each have their own space. Space for the humans to admire them and for the animals to just live.
If we don't try to invade their space, they won't invade ours. Simple, right?