To them it is completely obvious and real. That this life is a mere shadow, a simulation. Nothing will convince them otherwise.
But we (general public without that experience) can't possibly believe it, come up with many theories on what the brain is doing.
I've never experienced ball lightning but I can believe the ones who have most of the time.
Death experiences, I can't even imagine something more real than my current reality, how could I possibly start to believe it? Despite how obvious it is to those who went through it, it's impossible for me to even begin imagining something like that.