Because I was BORN an atheist. Two people I loved unthinkingly indoctrinated me into believing in something that didn’t exist. Key word – “unthinkingly”. The state of my knowledge at birth was the correct one.
Because religion, super-naturalism, has never explained anything. From the very beginning of civilization to the present, whenever religion has tried to explain previously mystifying natural phenomena (from lightning through mental illness to the size of the universe) it has always gotten it wrong. Always. It has not been right yet, and the odds are it will never be right, if we ever get to the point in human knowledge where we know everything.
Because religion is an inherently anti-human phenomenon. I’m a human, not a spirit. Religion explains spirits. There are no spirits, and there’s never been any evidence of spirits. As a human, there is a natural, logical way to treat other humans, and it does not involve burning them at the stake, making them believe what I believe at the point of a blade, or flying airplanes into buildings. Religion is cruel and inhuman, in almost all aspects of its justifying rationalizations.
Because I don’t need religion to be a good person.