Because it would mean acknowledging that colonization, slavery, and resource wars are universal human practices, carried out by all ethnic groups throughout history with varying degrees of success. From Black and Native tribes waging war against rival groups to the Mongols pushing into Poland, this reality shatters the leftist narrative that these are "evil" and uniquely 'white "crimes" requiring reparations, a narrative used to justify mass immigration and the humiliation of white people. That’s why it’s never discussed, and that’s why you’ll never hear about the Barbary corsairs when u was in school for example