I did not see it mentioned in the article but one reason delusions are so hard to treat is that they are an investment into a belief system and people are hesitant to discard that investment of time, effort, and credibility for fear of a loss with nothing to show for it. That line of thinking is a complete absence of objectivity due to sunk cost fallacy stemming from a behavior of loss aversion. Therefore new evidence, particularly conflicting evidence, are irrelevant.
https://en.m.wikipedia.org/wiki/Sunk_cost