why does anything "exist"? beacuse it can be completely described universally through our human senses, unlike god; to put it simply: the only reason you perceive your "reality" to be real is because you can taste, touch, hear it, etc. and so can everyone else, whereas god can exist in many different conflicting forms at once, and can only be "percieved" mentally.

well that's my really mindbending explanation anyway...I've got tons of others.