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.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.