Because peer pressure, ostracism and persecution threaten those who dare to voice a contrary opinion. This nation has created a climate that is hostile to nonbelievers, and because of that, few people are comfortable enough or strong enough to speak their true doubts. When they make us too afraid to speak up, they win by default.
Because people adopt the belief they were given by their parents at birth. (All babies are born atheist.) Children do not get to choose between all the available religious beliefs, or no belief at all. Religious belief is largely a question of geography. If you are born in the western hemisphere, it is likely that you will be born into a Judaeo-Christian family; if you are born in the eastern hemisphere, it is likely that you will be born into an Islamic or Budhist family. Pat Robertson and Jerry Falwell, prominent christian leaders, would be hindu if they were born in India. Even more importantly, children born into religious families are firmly taught not to question the beliefs they are given-- and this unquestioning mindset is often retained into adulthood. Few god-believers have deeply and honestly questioned the doctrines of their faith.
Because people haven't had an alternative for thousands of years. Since prehistoric times, religion provided the only conceivable solution to the mystery of our existence. Only since Charles Darwin proposed the Theory of Natural Selection in the mid-1800's, have humans had a viable alternative to Divine Creation. Even after then, the idea met with such resistance that it was rarely taught on a widespread basis. Only in this century have such teachings become widely available. Now science provides us with strong evidence for a natural explanation of the universe, without the need for any gods. But this information will take time to become universally understood and accepted.
Because people use religion as a means to support their particular prejudices. Discrimination of women, non-whites and non-christians finds support in the bible and its believers.
Because people have a fear of death, and want their existence to continue forever. This is a perfectly natural fear, and religion provides a safety blanket. But this safetly blanket has brought many believers to the point that they place much more importance on the afterlife than on this real life. Atheists choose to live without those comforting beliefs, choosing instead to live this life to its fullest while we are here. We think it is better to embrace the hard truth, than to be comforted by a pleasing fable.
Because most people have a urge to conform, and go along with what everyone else thinks or does. People also have a need to belong to groups. Many people go to church just for for the social aspect of getting together with others, and don't really believe what their churches preach.
Isn’t it more likely that the bible and every other religion arose from humankind’s lack of understanding about how the world works, and a need to feel important and loved? Isn’t it more likely that religion is perpetuated by priests and ministers who wield great power and authority over others? Ask yourself: why don’t you believe in the Greek gods? Because you regard them as the primitive myths of people who had a need to understand and create order in their lives. Atheists just believe in one less god than Believers do. If one religion can be an invented myth, then why not all of them?