Isn't declaring an unseen being non-existent just as crazy as insisting it is real?
I'm not sure what to do with this. This is usually asked by people who have never read a book, but I happen to know you're generally better informed than that. I can't answer without sounding like I'm busting out the Magna-Doodle, so forgive any unintended condescension in the tone.
It's possible to invent an infinite number of beings that may or may not exist. I will invent the Butt-Snogging Space-Flying Marble-Chewer as one example. Any similarity to phil is coincidental; I genuinely made up fictional characters.
From a purely statistical standpoint, it is unlikely that something I made up corresponds to something that exists in the real world. Therefore if someone asks me if there is a Butt-Snogging Space-Flying Marble-Chewer, the correct answer is to say that there is not one.
Now let's say the Butt-Snogging Space-Flying Marble-Chewer is supposed to have created the universe, talked to snakes, killed off hundreds of thousands of people, etc. The problem is compounded, so that not only is there no positive evidence for a Butt-Snogging Space-Flying Marble-Chewer, but there is evidence against his existence. In this case it is grossly irresponsible to say he (or she, or it, I haven't decided the Butt-Snogging Space-Flying Marble-Chewer's gender yet) exists.