Why do atheists think Original Sin isn't real?

[Esquilax]

The point is, even the little baby will at some point commit sin. There is a gravitation for humans to commit sin.

You have your causation exactly backwards, though. Sin as a concept was created to be unavoidable, so that the authors of the bible could convict everyone as guilty and sell them the cure to the threat of hell. It's not so much that humans gravitate toward sin, but that sin was built out of behaviors and thoughts that human beings naturally gravitate toward.