Because if you really think about it Gods Law is written in every persons heart.
When you do something wrong - believing or not believing why do you feel guilt? Who have you wronged?If there is no God then there is no guilt for any action or any reason to feel that way - but we all do believers and non believers.This is an unexplained mystery - explain it.Stealing , killing , hurting anothers feelings - there is no reason to feel guilt if all that you are worried about is man’s law - without God existing we should live like predators because thats what we really are if God doesn’t exist.Love becomes just an instinct and is meaningless.We are just instinct in our every action and thought with no freewill.Why should I care who I’ve hurt ,killed or stolen from as long as I get away with it. But we all know deep down inside that that is not true - that we do love and care how others feel - we have freewill and know when we have done wrong and as I said before - who have we wronged? And the truly evil people of this world are the ones who absolutely ignore what is written in their hearts and live that purely selfish life that I speak of above.
God does exist - its in your heart - all you have to do is search it and you will know the truth.