I've been reading this fascinating explanation of why leftists resist re-thinking their philosophy, despite finding evidence that refutes it. The author of the essay, Edward Feser, provides several possible explanations for the phenonomen. A short excerpt:
As the philosopher David Stove has argued, the modern tendency toward hyper-skepticism seems largely to be the result of a massive overgeneralization from a mere handful of cases where common sense turned out to be mistaken. Another philosopher, Michael Levin, has given a name to the peculiar form this error in reasoning has taken in modern thinking: the "skim milk" fallacy, the fallacy of assuming, in the words of Gilbert and Sullivan, that "things are seldom what they seem, skim milk masquerades as cream," so that common sense can in general be presumed to be wrong.