Well, if you assert that God exists, you must have proof that he does exist. You are right, God can't be proven or disproved. This means there is no evidence for or against God. Well, except the Judea-Christian God, considering the faultiness of the Bible. But that isn't the point. The hard part is trying to get Christians to realize there is no evidence for or against a God. But we ask for proof because you can't assert something without evidence. Therefore, when someone asserts that a God exists, we require evidence.
Also, simply because he can't be disproved does not sway one way or the other. You also can't disprove the Flying Spaghetti Monster, but that doesn't make it more likely that he exists.