Because there are anachronistic messages in both the Illiad and the Bible. Messages that even if it did made sense once, now they don't.
Well, to say that is to say that the specific messages - and all of them, not just the ones you don't like - were once true but no longer. Are you prepared to argue that this is the case?
Using one as a moral guideline is one thing . Using Mythological creatures as an explanation of human behavior is something entirely different.
According to what you said above, the "something entirely different" it is, is mental illness. Well, how do you define "mental illness"?
@frob: There are, indeed, positive moral messages in the Bible. Focusing only on the parts you don't like doesn't change this fact. I have seen more people motivated to charity than to genocide by it.