Actually, Agnostics claim either that it is not possible to have absolute or certain knowledge of God or gods; or, alternatively, that while certainty may be possible, they personally have no knowledge. Agnosticism in both cases involves some form of skepticism. Not a belief that anything could be god. So, the question, "Why assume first god does not exist?" My opinion is that we are all born with no knowledge of the concept of god and that the concept of god, sin, the resurection, the virgin birth, are all things that must be taught. Since these things are taught it is incumbant on the one being taught to "assume" this is true. Someone who is not taught something cannot assume it does not exist. Only those who have been taught and choose not to follow the teachings are able to assume first god does not exist.