atheism, by definition, is the lack of belief in a god or gods.
This and agnosticism, by definition, is a person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena; a person who claims neither faith nor disbelief in god, I believe are default positions depending on the individual.
Without any cultural or religious influence, you either believe in scientific reasoning and disbelieve in a deity or you are simply not sure. So they are, in fact, religious default positions.
That isn't to say people don't go from conforming to a religion to atheism or agnosticism.