Because like most things in this world, it isn't ABSOLUTELY true, it's only a mental construct to describe certain things we observe.
Children still need raising. Obviously, we aren't completely hard-wired creatures. And, humans have free will. Even when we know right and wrong, humans can and do chose to do wrong, for many, mostly selfish reasons.
I've never read the book in question, but I have read another, wonderful book, called "The Moral Animal" by Robin Wright, that apparently deals with many of the same issues. I highly recommend it.
So have I!