Because you can't prove it exists, and the Burden of Proof works this way:
If you say that something exists,
you have to positively prove that it exists. Not only that, but in order to prove the existence of the thing you claim, you have to show that your proof can
only prove your own claim.
For example, if I told you a Ferrari peeled out in my driveway, it's not enough to point to tire tracks in my driveway -- I also have to prove that those tracks could
only have come from a Ferrari.
So:
1. Prove your god exists,
and
2. Use evidence that can
only be evidence of your god, not evidence that could have been created by natural processes.
So
you decide how it works?
Nope. I do know how it works, though.
You claim a god exists. There's no evidence of the existence of a god; that is to say, there is no evidence that can
only prove the existence of a god, rather than being proof of the mechanistic processes which generated the evidence.
On the other hand, proof that god
doesn't exist has already been provided to you via an argument from impossibility. Here it is again:
An omnipotent, omniscient, and benevolent god
cannot exist in a universe where evil also exists.
If there is evil your god doesn't know about, your god is not omniscient.
If your god knows about evil but can't stop evil, your god isn't omnipotent.
If your god knows about evil,
can stop evil, but
doesn't stop evil, your god is not benevolent.
Your god is an impossibility. Nothing impossible can exist (if it could, it would not be impossible), therefore your god cannot exist. Therefore, your god does not exist. There's the proof.