It interferes with their easy assumptions that everything is either black or white, one is "with us or against us", and "things have always been this way and always will be".

It's why "flip-flop" was made a pejorative—the progressive concept that a person can hold one opinion and then another after learning more about an evolving situation makes them crazy because it undermines the religious concept of "unshakeable belief". (Which it actually does not; learning from mistakes is one of the tenets most religious parables teach. They often eagerly conflate "faith" with "personal opinion" because they like to have faith. Lots of it.)

Also, thinking is hard and revisiting a situation objectively takes time. They'd rather go with what they know, which was learned from their parents, which has always been "the way it was, the way it is". Shades of gray are too complicated to explain to their children.

They'd much rather shoot first and believe there was no good that could have been done because that lets them sleep at night.