> Uncertainty is ubiquitous, yet few people feel a need to qualify their disbelief in leprechauns, unicorns, Darth Vader, etc. Why is it only gods where this seems to be required?

It's because those things would by definition be within our universe and thus probably testable. However a deity that controls our universe may be outside it, thus leaving us with no way to test for him, despite his control.

In hacker analogy, would a Sim character in a VM be able to be completely sure that you (god) don't exist?

To "not believe in god" is rational based on available evidence. To "believe there is no god" is an act of faith in itself because it (very likely) cannot be tested.

At least IMHO.