It's important to be clear what we're talking about. I'm using the following definitions:
atheism - the belief that God does not exist
relativism - the belief that no one point of view is more valid than others
agnosticism - not being sure whether God exists or not
When two or more people have a discussion or an argument, they are usually coming from different points of view. Each of them will have their own experiences; each of them will have their own backgrounds; their own reasons for believing what they believe. Sometimes one point of view will be "better" or "more valid" than another For example, the view of a professor of engineering on how a car engine works is probably going to be better than the view of a six-year old child, because they have had more opportunity to become well-informed, and to experience the car engine itself.
But sometimes neither person is in a better position to know, and then it just comes down to people's background, what they assume and what they believe. If we find ourselves in that situation - where the only reasons for disagreement are background and assumptions, then it is a good idea to admit that at the end of the day we don't know, we can just have an opinion.
If God doesn't exist, then no-one can have a "better" or "more valid" experience of God than anyone else, which means that we are all groping in the dark. We could have two people meeting one another and one of them believed in God, and one didn't believe in God. Neither of them could have any real evidence for their claim; they would both be believing things entirely on the basis of their backgrounds and assumptions. But that means that we couldn't tell who was right.
At the end of the day, the only way that Atheism cannot explain why many people do believe in God is by saying that people sometimes believe wrong things because of their experiences, or because they want it to be comfortable to them, or something. But whenever they admit that, they are cutting the ground from under themselves, because if those reasons can make other people wrong, it can make them wrong too.
If an atheist is being logically consistent, they can never be sure. They can never say categorically "God does not exist" - it can only ever be "I can't see how God can exist" or "I don't think that God exists". But I wouldn't call that atheism; I'd call that agnosticism, as it's saying that at the end of the day, you're not sure.