I do not believe in God because: There is no proof.
I do not believe in fairies, hobgoblins, wizards, orcs, leprechauns, unicorns, minotaurs and lizardmen for the same reason.
Why is it not questioned that the latter group of creatures are mythical, but God - for whom there is the same amount of proof as there are minotaurs - is believed in by so many?
I think the onus is on the believers to proof there *is* a god, rather than atheists to prove there is not. If this is not the case then should we also assume everything else we can imagine also exists until proven otherwise?