If the metaphysical can't be proven, how can we assert anything about it?
This is much like the "I've seen something in the sky I can't explain, better attribute it to a UFO or aliens" rationale.
The first element to it is that as children in North America, especially more than 10-20 years old, we've been taught that religion is truth, in whatever subjective form comes from the parents or churches we hear them from.
Second, we're told that religious elements are miracles of some type, "God" is invisible, his human form "Jesus" can only be seen to his followers, none of these intangible things can be tested, but the logic behind them and the book that is written that contains things attributed to the real world is tangible enough to falsify thus put to the test of whether or not it's true.
Thirdly, someone already mentioned:
Religion is for those that believe in hell, spirituality is for those that have been there.
If I had been raised in a society that did not lie to me about the existance of the:
I might have been able to believe in a supreme being.
.. and this is called control. The control associated with these made-up things is obvious, and it is just as obvious with religion when objectively comparing it.
Most people rightfully fear death, religion controls that fear and provides a comforting answer, at the same time, adding another fear element that should you not choose their path you will be subject to a type of punishment you've likely never experienced.. thus creating fear of two unknowns -- death and eternal punishment. It's common wisdom that people afraid are easier to control, so add fear of those two unknowns with the comforting feeling of eternal life after death, as well as going to heaven, and you have yourself a wonderful life of self-delusion which won't ever get proven wrong because you'll be too dead to figure out the truth.
Anyhow, as an agnostic, I recognize one thing -- I don't know whether or not there is a creator. On the other hand, I know the mythological creators, whether it be from what we call "mythology" in contemporary terms or the religions people preach and practice, they are given tangible qualities that are easy to debunk with reason alone. The fact that the books associated with these religions in particular that I have in mind were created by people, who subjectively wrote them, subjectively decided how to interpret them, transliterate them, etc., I know there are far too many facets, layers, and people involved in describing things in these books over a long period of time to be able to accurately depict them, there's far too little tangible evidence that these stories involving tangible things happened.
Any analytically objective person pertaining to the subject of religion should outright reject it, and any person not falling to the absolutist game (i.e. falling into a logic trap where the brain wants a conclusion even when there cannot be one) that besets atheism should admit they don't know rather than coming to a convenient conclusion that can, "conveniently" change the same way religion does as time goes on. Science is one of humanity's greatest accomplishments, but logic dictates people, being people, will bring religion with it, so the battle of objectivity and non-absolutes wouldn't end even if the Muslim, Christian, Sikh, Hebrew, Scientologist, Hindu, etc. died out.