Why do atheists think they can know if a god exists on the other side of the universe?

I don't need faith or a space ship to know that god is not real. Not when it is easy to demonstrate how the whole mythology was made up in the first place. Not when attributes of god are easily deemed false. Not when there is mountains of evidence showing why many of the bible stories are impossible. Not when "god" has exactly the same prejudices and ignorance of the people speaking for him. And most certainly not when he simply serves absolutely no purpose.