why not believe in God!

[Entropist]

If you believe there is no "God" that is called a belief system.

The absence of belief in gods is in itself not a system. Because of atheism, different kinds of systems may arise out of that lack of belief however: secular humanism, nihilism, existentialism, objectivism, materialism, anarchism, communism, transhumanism, naturalism, postmodernism, etc etc etc (this is why even atheists here on this board have divergent ideas here after all)

THOSE are "belief systems" all of which may (or perhaps may not) include the absence of belief in gods. And they are all very different belief systems. Atheism, taken by itself, is quite boring and bland.