Deliberate, Self-Inflicted Amnesia
At some point, every culture across every age in recorded human history has defaced historical monuments and symbols and attempted to erase and/or rewrite history.
Ancient Egyptians did it pretty regularly, with the change of each dynasty. Early Christians did it, replacing pagan temples with Christian churches. Muslims have always destroyed "infidel" monuments, right up to the present. The Mongols did it across Asia and Eastern Europe. African tribes did it, Europeans did it in the New World, and American Indians did it, also.
There's just a backwards, self-destructive human tendency to obliterate monuments and flags and symbols and scriptures of prior (and usually more advanced) civilizations, in spite of the loss to history.
However, one of the things that most distinguished the United States from every other nation in human history was our dedication to PRESERVING our history, lest successive generations forget the origins of our founding principles. It was really an evolutionary step out of the dark ages, to actually preserve that which went before us, both good and bad.
Today, we see the backwards human tendency to destroy history rearing its ugly head once more, this time in the guise of uneducated and vengeful "Social Justice Warriors," who offer only condemnation and destruction of the past, regardless of the loss to human memory and understanding.
Every race and every culture on earth has its own skeletons in the closet. Every race and every culture is guilty of inhumanities at some point in the past. If we go seeking modern "justice" for the crimes of our distant ancestors, then all human progress will dissolve into another dark age of ignorance and meaningless civil war.