Because, as you so relentlessly claim, we should always adhere to reality, stick to the evidence, and be prepared to back up spectacular claims with facts. The claim that deities don't exist is at least as spectacular as the claim that they exist. Big claims demand overwhelming evidence, as you always say. Theists can never find evidence that will sufficiently satisfy unbelievers, and atheists cannot provide evidence of the inexistence of a deity that will convince theists. Since you so like Ocam's Razor, the simplest, verifiable claim that can be made about God (or any deity that you chose to elect) isn't that it doesn't exist; simply that it is absent.
Eden