That's very simple: because we need it to be that way. Somewhere along the line, humans figured out that it benefits each individual to work together and they quickly figured out that if we allow people to steal from each other, murder, rape, etc., the whole thing wouldn't work. I don't see any reason why humans couldn't establish these morals among themselves knowing that the entire human race benefits from the following of these principles.
But it already is that way. Because as it is, whether it's aknowledged that morality comes from divinity, cultures still differ on what those divine laws are. Christians say God wants one thing that Muslims find to be atrocious and vice versa so who's in the right? Placing a divine influence on morality doesn't come close to removing that cultural problem as you explained it. In fact, if anthing, it makes things worse.Without a larger law above and beyond human opinion, we fall into the trap of having to allow other cultural practices that violate what we morally believe. And, since both cultures' laws were made by men, then neither has priority over the other. As such, we now must allow atrocity.



). Your attempt to reduce my belief to a "security blanket" (and thus trivialize it) doesn't change the validity of my point.