Human nature. We’re hardwired to form tribal groups around perceived similarities, which necessarily entails excluding people who are different. Overcoming the negative results of this biological wiring is an eternal challenge for us.

Edit: downvoting me doesn’t make the truth go away. This is established science. Humans will form groups compulsively, over literally anything.

https://youarenotsosmart.com/transcripts/transcript-tribal-psychology/

https://www.simplypsychology.org/social-identity-theory.html