This question is better suited for an anthropologist or sociologist than a historian. The reason being that this question is less rooted in historical study but rather sociological or anthropological analysis. Historians try to deal with specific instances or subjects and, usually, broad questions of human nature are outside our purview. For instance, you could ask "Why was the Third Reich racist?" and we could address that as you name a specific entity within a time frame. But something as varying as racism over thousands of years of recorded human history is too broad for a good answer.

Some time ago I assembled a list of earlier posts about the history of racism, which I can post here below:

• How would an Egyptian in Alexandria at the turn of the millennium have perceived race and skin tone? by u/cleopatra_philopater
• Did the Prophet Muhammad (pbuh) ever say this? by a now-deleted user
• Why are "Moors" often depicted as black in European art? by u/sunagainstgold
• There's been a lot of talk here about how medieval Europe was more racially diverse than we think. Would it be out of the question to see a white person in 1000s-1300s Sub-Saharan Africa? by u/labarge3
• Europe had normal diplomatic relations with non-white nations before turning explosively racist to justify their actions against all others. What happened to cause this shift? by u/victoryfanfare
• How do contemporary academics who say that race and racism date to the Enlightenment explain what seem to be examples of racial categorization, racial stereotyping and racial prejudice in ancient literature and art? by u/restricteddata and u/PhiloSpo

This you can read, especially the last two, while waiting for a new response from one of our users.