Human beings are quite rational. But they are not rational in the classical economics sense. Human beings recognize social aspects. They also have their own social status they want to improve. They also know that they can exploit their social status and gain advantages, favors, mates and power over other human beings. They know that openly backstabbing one of your key players may discourage other ones from obeying your power, so they let others do the stabbing take the blame.

Yes. I've seen people attempt to argue that the existence of racism/sexism is proof of the inadequacy of the discriminated group, because otherwise it isn't rational and humans are rational. Which is either a concerning level of trust in economic theory or a disturbing use of economic theory to justify existing bigotry.