Why can't modern liberals think outside the box of political correctness and cultural Marxism?

"This is the way the world ends
Not with a bang but a whimper:" T S Eliot

Why can't modern liberals think outside the box of political correctness and cultural Marxism? I think it's more a lack of courage than the fact that they were taught that nonsense from grade school through grad school. The lack of courage seems also to be based in a lack of physical courage, caused by a sedentary life full of technical gadgets and passive viewing of a pervasive liberal media.

Many liberals were intimidated or saw others humiliated by more knowledgeable teachers and professors for thinking outside the liberal box. Real truths about human nature are not even taught in our schools, for example, the synthesizing science of sociobiology is left out of the humanities, or is belittled when it is rarely mentioned.

Sports can teach courage but even our sports have been increasingly invaded by passive liberals who seem to prefer losers to winners, at least on the surface.  And of course the military is the traditional way to teach courageous behavior.

Being poor can teach courage in order to survive, but it can also teach lying, cheating, and criminal behavior.

I suppose courage can still be taught in the family even if the culture does not teach it. We can reward our children for showing both physical and intellectual courage---but showing courage in an amiable way, if possible.