It's true that the question of God's existence is epistemologically fairly trivial and doesn't require its own category of justifications, and it's also true that even many atheists don't seem to notice this. But even with that in mind, it almost never actually helps in convincing people to become atheists (most theists won't respond to a crash course in Bayesian epistemology and algorithmic information theory, but they sometimes respond to careful refutation of the real reasons they believe in God), which is probably why this point is often forgotten by people who spend a lot of time arguing for atheism.