It might well be proven in some contexts. But as it is a claim about the past, it is bound to be merely the most likely reason for religion: We do know that religion is psycholgically useful, and useful in societies, and we also know that there is no proof whatsoever that there actually some godly being, which, idk, implanted the idea in all our heads, or conjured up a particular document.
With that, the most likely explanation becomes that man invented religion.
As far as why there's no proof, I said this in another thread, but for the sake of documentation:
The reason why there's no proof is because it's not possible to prove the core concept of most religions: That of the existence of some sort of godly being.
For, according to the most common version of god-concept I've heard, there's not enough information even be able to tell what a possible proof of the claim (that the godly being exists) would look like.
If, for example, someone made the claim that, say, the planets moved according to Newton's theory of universal gravitation, I could take that data, and tell anyone who asked what would evidence for that claim, what might prove it, and what would contradict it.
The most common version of the god-concept being so vague and insubstantial, you cannot even do that, much less actually find and demonstrate a proof.