Here's your "WHY" -- because everyone inherently desires human contact and communication, wants to be seen as a valued member of their community, and doesn't want to do anything that would jeopardize that, such as breaking established social norms, mores, or taboos.
Your anthropological/ historical assessment is a nice nutshell.
But its missing the point I was making:
-Those very social norms, morals, taboos etc. themselves need a 'why'. They are not at end in themselves.
For instance, take murder. Why is it wrong?
Lets ignore all circumstances as to why a murder might occur, or concequences that might arise out of it. Lets reduce it right down and focus just purely on the act of X killing Y.
Like I mentioned above- assuming there was nothing outside the act itself, there were no negative concequences at all from X murdering Y: the Athiest cannot say that murder is bad, as no bad came of it. You might say "murder is bad in itself"- and you come back to why is it bad in itself?
The argument can't proceed any further.
On the other hand, if you follow that path of reasoning, a christian could take it one level further, and say "because God said it is wrong".
Or consider this: what if the murder of Y by X resulted in nothing but GOOD concequences. How would an Atheist argue that the murder was wrong? If anything, a true Atheist would have to admit that in this case, murder was a good thing.
You could convincingly argue this is a good thing, as it allows a society/ person enough flexibility to allow what is normally seen as wrong. eg Killing someone like Hitler would be a good thing to most people.
The problem is the "slippery slope" - where does this subjective reasoning end?
To a true Christian, even the murder of someone like Hitler would be immoral.
This fundamental flaw in Atheism (the lack of a fundamental moral framework) is what fuels the stupid opinions of Fundamentalists- there are great examples of this line of thinking gone too far in the link of the original post on this thread.
Yes, society has decided over time that it is inconvenient/ irresponsible/ dangerous etc. to allow murder, but this doesn't answer why it is fundamentally wrong/ immoral- that's typically where religion steps in to give the 'why' that humans crave.
The point of difference in our arguments I suspect has more to do with wether the religious beliefs stem from the necessity to create common understandings of what is right and wrong, or if religion informs the laws of society to begin with.
It's in danger of becoming a chicken or the egg argument I'm afraid, and one that is grey rather than black or white.
This is similar to science, in that you could possibly explain everything right back to the big bang, and even if science could explain how and when exactly the big bang came to occur, it would never be able to explain why it occurred in the first place. Again, God steps in to fill that void.
The greatest minds of our time, including Newton, Einstein and Hawking all allowed for/ believed in God.
I say let's focus on what I think (I hope) we can agree on - that fundamentalists of any kind are fools.