But it can be proven: that’s what science does. We perform experiments to verify in multiple ways that the theories about the world that we’ve developed are consistent with the evidence.

In contrast, if you believe that e.g. the machine elves some people see on DMT are real, there are all sorts of experiments you could do to test that. But not many people bother to try, because at some level they know they’re not real.

Of course, you can speculate that these mental realms are a different aspect of reality and not subject to the same constraints, etc., to which the obvious answer is that’s right, they’re called fantasies or delusions.