This atheist doesn't. This is a conclusion, not a premise.
I know many believers start with the conclusion and work backwards, manufacturing whatever facts and reasons they need to keep that belief intact. But don't project that failing onto everyone else.
It may have been a conclusion at some point, but at this point it's a premise.
As I said, I have my reasons for believing that God is real. It's a premise for me at this point, but I know that you have a different interpretation of the same facts that led me to believe that in the first place. I believe God exists because I believe that God is a better explanation for all the order and structure in the universe and in earth's ecosystem than random chance. You have a differing opinion.
Perhaps I should restructure the premise slightly: the premise that most believers work from is that God creating order and structure from chaos makes more sense than the alternative - random chance creating order out of Chaos. Atheists work from the premise that random chance creating order out of chaos makes more sense than God creating order out of chaos. So regardless of your conclusion, there's still a premise that you accept before drawing your final conclusions about God. But once you've drawn your conclusion, it becomes part of your premise for approaching any evidence pointing to God's existence.
For example, you staunchly deny the witness of the thousands who saw Jesus after His resurrection, and those who watched His ascension to heaven, which would certainly be strong evidence. Your denial has no merit except for your predetermined idea (premise) that God is, in fact, not real and the Bible is just a story written to try to convince people that He is. Your view (that God does not exist) determines your analysis of the evidence.
The same is true of science. Where a creationist (which I would call anybody who believes that God created the world) looks at the universe, the earth, and all the order and structure in it and sees this as evidence of God's handiwork, an atheist looks at the same structure and order and sees it as evidence of the amazing effects of random chance. The analysis of the evidence is predetermined by a premise (perhaps once a conclusion) that it makes more sense to believe that random chance resulted in all this order and structure than that God exists and created it.