Because religious people needed a word for the unimaginable absence of their favourite hobby in other people. Non-believer was already used for people who believe differently relative to them, so a different word was made for those who are even worse and defy divinity. It is useful to sum up your stance towards religion for religious folks but indeed carries no meaning in non-religious contexts.

What is termed “militant” atheism, where someone argues against religious ideas or actions, wouldn’t exist because there wouldn’t be anything to argue about. But atheism itself would exist as a default, and already does in reality. No baby is born with a belief in any religion. So the only thing that changes in your hypothetical is the baby never gets indoctrinated with such beliefs.

The word probably wouldnt exist in a world where atheism is the default