Because humans opt for what's easy, shortcuts.
You have two options, one is to accept a person is far more than a single opinion or interaction, understand their point of view and share ours with an open mind.
Or dismiss them because we've already decided what's right and wrong and gotten emotionally invested in that distinction, it makes people feel safe, dualist thinking just like our ancestors.
For our ancestors, that speed in decision making had a survival function. It was very essential to distinguish good and bad; life threatening and not life threatening. The fear was purposeful.
