It's not their fault. They believe in demonstrably false things. They know they believe in demonstrably false things. Lies are the only thing they can use to defend their beliefs, because the truth says otherwise.
I think in some cases it boils down to the idea that the level of conviction of a belief has an actual bearing on the veracity of that belief. They would never claim that what they know is demonstrably false - the strength of conviction of the belief shows that it's true. Works pretty well with use of the Force, so it may as well work for determining reality