Because God is the One who brought everything into existence. We wouldn't draw even one breath without God allowing it. Without the God of the Bible, we would have no foundation to even reason how someone can exist if they were not created. Therefore, by the very fact that we can reason anything (arguing for or even against God), the God of the Bible must exist. It's a bit more complicated. I can find a couple articles about Reason and Logic and God, if you'd like.
But that doesn't explain how God can exist without being created. That's like telling someone that the sky is limitless when in fact it ends about 60 miles up. That's where the last air molecules are. It seems limitless and nowhere on earth can you see where it ends but like everything else it does have a beginning and an end. Even time begins when God created the universe and ends at a certain time as mentioned in scripture..
...and I FOUND THAT THREAD! It does exist after all...
http://christianchat.com/bible-discussion-forum/133112-who-made-god.html