Answer by Anthony Sargeant:
The simple answer is because God does not exist in the sense of an external being – there is no external "him" or "her" "out there". There is only us. God is a 'construct' of a social animal, homo sapiens, and that 'construct' deludes the individual animal into patterns of behaviour which ensures that the gene survives – the individual's tribal gene that is – even at the expense of other tribes or sometimes of the individual herself or himself.
As long ago as 1842 Feuerbach noted that the secret of all theology was anthropology – and that religious beliefs were nothing but the projections of human qualities and human values onto a (non-existent) deity. We now have a better understanding of genetics and social and evolutionary biology and can put Feuerbach's observations into a broader perspective.