Do you claim to know for certain that God does not exist?

So if we cannot know with a high degree of certainty that anything is real, why is it that many people dismiss the possibility of the Flying Spaghetti Monster being real? Is it because the Flying Spaghetti Monster is impalpable? How does that prove that It is not real? Is it because we cannot physically see The Flying Spaghetti Monster with our eyes? What about opening that third eye?