1. The mind always tries to simplify. One way that manifests is in dogma. Many people are satisfied or just dont have the time and energy to continually update or critically assess their own beliefs (not that this is justified, just an explanation). This is the entire reason formal science education exists at the highest levels. We didnt evolve to be critical or rational. But at our best we can learn to approach it collectively.

2. Many people think that what they learn in school or from other authorities is immutable fact and that's the end of discussion. This is an unfortunate failing of the education system.

People are people. Take away any differences in appearances, beliefs, communities, etc. and people will still find reasons to divide up and seek to destroy the outsiders.