Why? Unless the experience displays obvious entanglement with external facts they couldn't have known at the time, "it's all in my head" is a perfectly good explanation.

Unfortunately, there's a slippery slope from "it's all in your head" to "it doesn't matter"/"you're making it up". Look at the history of psychiatry and mental health for examples.

Not saying you're wrong, just that there may be layers of complicated cultural biases preventing people from accepting that answer.