The problem is that as soon as an idea is postulated, the people who postulate the idea immediately call those that do not share the idea as Those Who Believe Idea A is false. Ideas, That There Exists A Deity included, are defined by those who hold them, not those who do not hold them.
I do not hold no claim as to whether or not a god exists. I doubt one does, but for all I know, it might be true, specifically because there is as of yet no way to disprove it. But when I take into account all of human history, and the history of the universe as a whole, read ancient books and the like, I begin to understand that all things have some origin point. Before the foundation of any kind of religion or spirituality, when Homo Sapiens Sapiens was just beginning to achieve a semblance of modern sapience, would they have called themselves Atheists?
Of course not. Atheist is a religious label. An outside label, that can be applied to anyone by anyone with any religious attitude. The Romans called the Jews and Christians Atheistic, for failing to adopt the Roman pantheon.
It is not that there is a strong belief among atheists in the non-existence of a deity, it's just that once people come up to you and try to convince you of an idea, and do so frequently with no success, there comes a point where one must defend his or her position.
There exists a teapot in orbit around the sun, between Mercury and Venus. Why don't you believe in it? I just told you it's there. Are you an Ateapotist?