Because the concept of "proof" is human and mostly tied to science and reason, while god is tied to faith which is belief despite the lack of proof and thus requires no reason.
I conside myself an atheist(yes, an atheist that practices occultism and other sorts of esoteric and spiritual paths is an irony and that isn't lost on me) but I do not exactly tell others that I don't believe god exists.First because people can believe whatever they want, so long they're not hurting others with their faith, and second because it's more like it doesn't matter much to me.
I have two views of god, one is that god is just an energy of sorts that exists and permeates everything, that energy does not have a consciousness but it follows a hidden directive that makes it do what it does(whatever it is) and because the thing called "god" is an energy and not a being, it cannot be interact with the way you would with either a sapient or non-sapient lifeform.It just is, the way the alleged dark energy is.It exists and it may be used in a way if figured out it's properties.
The other view is that god isn't this powerful figure(because if he was then there would be a myriad of questions regarding the other claims made of him), but more of an entity that has some measure of power, so not really the omnipowerful and omniscient creator-being the Bible speaks of, but just a being above humanity.The most apt comparison I can make are ants and humans, which in this case god would be a human while we would be ants.
In both views, a person would need a direct interaction with god for it/him to be proven to the person.But that is one person experiencing that, how to make others experience the same?They can't, because perception is something of the self and no one else can perceive the same as you even if you're both seeing the same thing, because our perception is influenced by our understanding of reality, our personality and our mindset.So how do you create proof that will affect everyone in the same way?Each person would need to have proof tailored to their perception, and so far I do not believe someone(or something) would be capable of that on such a huge scale.
So god will stay a subjective talk subject, rather than an objective one.