How can one not believe in god as the root cause of the universe?

How can you lack belief in the existence of god? I define god here as prime cause. As the world is a sum of collections of events, causally linked to the past through time, then there must be a prime cause. As of now the Big Bang singularity has been discovered, but to say this Big Bang occured in nothingness, where there is no volume, no time, no energy, completely nothing...isn't it a bit far fetched? There ought to be cause(s) to this singularity, and cause(s) to that cause(s). In the end it should still lead to god.