Because nearly all things that could exist, don't. When you're in a state where you have no evidence for an entity's existence, then odds are that it doesn't exist.
Suppose that instead of asking about God, we ask "does the planet Hoth, as portrayed in the Star Wars movies, exist?" Absent any evidence that there really is such a planet, the answer is "almost certainly not."
If we reverse this, and ask "Does the planet Hoth, as portrayed in the Star Wars movies, not exist?" the answer is "almost certainly."
It doesn't matter how you specify the question, the informational content of the default answer stays the same.
I don't think that the Hoth argument applies here, because what we're looking for is not just some teapot in some random corner of the univers—it's a God actively involved in our universe. In other words, in God does exist, He's a very big part of our existence, unlike your teapot or Hoth.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.