They were allowed to fall under a great delusion because they didn’t love the truth. It doesn’t really answer the post’s question, “How in the world can this person not see the truth?” The great delusion can explain why the person might double down on mistruths, but it doesn’t explain why the person didn’t value the truth in the first place. I think it’s because they made idols of their own egos. Acknowledging that a situation has degraded means admitting that one hasn’t succeeded in preventing the degradation. For example, Silent Gens and Boomers are often delusional because it would hurt their egos to admit the truth that Western Civilization has been degraded on their watch.