Judaism has always been about more than a belief in god. It is a nation of people that was established in Israel/Judah. Were all the Greeks required to believe in the same gods? Were all the Romans required to believe in the same gods? Since when has any nation of people been required to believe in the same gods or any god at all?

It is now the 21st century. Just as most people living in secular cultures have realized that there is no god, so have Jews. You do know that there is a high correlation between high intelligence and atheism, don't you? There is also a correlation between belief in a god and socialization and peers in any country. Ergo, where you grow up and what you are exposed to growing up determines your belief system.

Why would Jews be any different?