I am not so sure about this. Like it or not, atheists enter a world dominated by religion and god-believers. Therefore it seems that the stance of atheism, of not believing in a god, is a reaction to theism. If theists left them alone then no talk of atheism would arise.
In the same way, a person who tries to be a better person is doing so out of the supposition that he is not already a good (enough) person. He is actually acting as a participant on the same coin as one who is not a good person. When we are born into this life we are born into a place where god, religion, and all sorts of ideas are in the air, so to speak. They are unavoidable and they are invasive and insidious.
It's not necessarily that "everyone knows God exists," but rather that so many people have filled the atmosphere, over millennia, with the presupposition that god exists. It's nearly impossible to find anyone who comes into this life unaffected by this prevailing idea. So atheism is as reactive as religious ideas are.
The minds of so many — perhaps most — religious people are indifferent to the feelings and understanding of others (in regard to how others feel about god and belief), and they are often unaware that there can possibly be any other way to go through life other than to believe in god, Jesus, the supernatural, etc. This is the reality that the atheist faces in society and therefore the atheist finds himself refusing to believe in a god.
When I was just out of college I got a job at a newspaper in a small Southern town. When Christmas came around everyone planned their party at the office. Not being a part of their mindset, their belief system, I did not want to take part. Before the Christian holiday I had not even been thinking of Christmas; I was just going about my happy days. It was unthinkable to the others that I did not think the way they did. My disposition was literally unthinkable to them. So instead of simply being happy with my own point of view and existing in peace, I found myself in the position of having to react even without trying to make an argument. We cannot ignore the reality of other people's ideas and that they affect us and push us to take a stance.
It may even be argued that the human brain, collectively, has been conditioned by religious beliefs over the millennia in the same sense that they have been conditioned to evolve in myriad other ways.
When I read what atheists have to say it seems always to be in response to what theists assert, most often imposing on the former group. So, yes, atheists do not accept God and this is because they are not accepted for having a mind independent of the religious point of view.
When you write, at the end of your comment to me, "They deny that there's any good reason for thinking that God exists..." you seem to be saying the same thing that I just explained — the denial is a refusal to believe, is it not? And what are atheists refusing except for a principle set in front of them that they must deal with.
