Theory:
In our ancestral environment, the link between bad idea and failure is obvious and we adjust our psychology accordingly in order to survive. In our complex modern environment, the link between bad ideas and failure is not obvious and so they are retained and proliferate.
Conversation
BONUS theory:
This absence of an effective regulatory feedback loop for our ideas is partly what makes philosophy such a no-man's-land, and conversations between competing ideologies so difficult--no one can effectively prove or falsify anything.