I figure this is because to have animals, they'd have to have conciousness. And as ours is not a pre-existing world (it was created by and for us), we'd have to create or recieve any conciousness. Right down to bugs. Which means there's absolutely fuck all 'alive' in our world except, well, us.

Same for us. It's kind of unnerving, because there's a pretty big outdoors area now, and still devoid of anything 'alive' (if you don't count plants) besides us.