I’m an atheist because it is natural to be so.
In every time, people who were not subjected to irrational fears and intelligent enough to be free from social conditioning were atheists.
That has been true for a very long time: the first documented atheists were Greek philosophers about 2500 years ago.
Through time, when social conditions let life be bearable, people have found they don’t need a god to fear or invoke in battle.
So, let’s say everyone is being born an atheist, and so he/she remains as long as no one makes his/her life miserable by terrorizing her/him.