Because they don't know everything, because they can't replicate everything, because not everything is stable, especially in the fields of consciousness.
Scientists often deny what they cannot replicate; however, centuries often pass before their comrades are proven right. In the fields of consciousness some are pioneers while others are hopelessly locked into their dogma.

To me it's like a religion, a specialized religion that leaves out all other religions.
Because they only know THEIR religion, one that is bounded by their own pre-conceptions that gives them comfort by replicatablity.

Lindy