'God exists' and 'God does not exist' are equally valid, as neither can be disproven 100%. I use my free will to come down on one side or the other, otherwise you'd never be able to have an opinion.
And my choice is to come down on the side of God not having created the Earth 6000 years ago. However, I do not just pull this out of the air, I accept that the evidence suggests that the Earth is not 6000 years old, but billions of years old.
So I use scientific method to break the stalemate, and play the odds. And the odds tell me that there is no God, on the basis that the basic claims made in the Bible have been shown to probably be not 100% true.
In short - it is more likely that the Earth is billions of years old because this is what the evidence as we can best understand it points to. Why would I go against that and believe in God?
I will believe what is more likely to be true, also if I can avoid a supernatural explination for something I will, because you can observe scientific happenings all the time, you can not observe supernatural happenings, unless you are one of those weird people who claim to have seen a ghost, spoken to dead people, or most weird of all - had a cosy little chinwag with a big magic man who lives in the sky, who made all the frogs and stuff that live on our planet.