Because their "thinking" is actively discouraged, and "evidence" means their bible. One of the last pair that visited me was a pretty young lady with glassy eyes that didn't respond to anything. Frightening. The other time I've seen that is with a Hare Krishna body that had clearly abdicated from making any decision.
Most effective and most amusing way is to get them into a discussion in which they have to explain what their source book says, and justify why it is a good thing.
"God praised Lot as a good man. Given Lot's previous behavior as described in the bible, if I behaved the same way would you regard me as a good and honorable person?"
"Do you aspire to be a disciple as specified by Christ? I.e. not loving, but someone that hates everyone around them" (Luke 14:26)
Or, of course, any bits from the wonderful diatribe in The West Wing
But this is somewhat OT, so it is unlikely I will continue this conversation!