"How can you be smart enough to discuss the topic X intelligently, and yet dumb enough to not notice that the tribe X is losing the fight and you could have easily joined the winning side instead? How can a person so epistemically rational be so instrumentally irrational?"

I doubt that such a calculation is in any way conscious, but behind the scenes, something like that is probably happening. Truth detectors for "socially advantageous" are probably stronger than those for "predictively accurate".