Why do atheists think Original Sin isn't real?

[Rev. Rye]

If you're born a human sooner or later you'll commit a sin. What's there to argue about?

I'm personally inclined to think it's an idea that's got a point, but it's problematic to me that it's connected with religion, namely that:

• It presupposes a deity (which is nowhere near conclusively proven)
  
• It can somehow coexist with a God who is all-knowing, all-powerful, and not a complete cunt.
  
• The idea of sin promoted by the same people who tend to promote the ideology is itself deeply flawed (to varying degrees, depending on the emphases done by any given idealogue). Not helping that it's often used for social control and not an honest attempt to help people actually try to overcome it, just not express it.
  
• The (sometimes) tacit assumption that this God can help with that even though A) He either created it or allowed this to happen, B) Empirical evidence suggests this is not even close to the case.
  

Personally, I've actually said on some occasions that when I left the Church, I still retained a belief in something like original sin, because, given all the insane shit people all over the world do, it's not too hard to think that there's something inherently messed up about humanity (fun fact: humans are social animals, and yet, throughout millennia of our history, we've had an astonishingly hard time trying to build societies that can actually function properly, especially if more than 150 of us are involved.)