Well for one, many of their political beliefs (including the ones littered with absolutes and nonsensical animosities) were basically coerced into their subconscious thought processes at a young age before they could develop the critical thinking skills to resist and question those beliefs. As a result, they went all through life viewing the world from narrow but penetrating perspectives that caused them to scour acutely for evidence that supports their beliefs with telescope-precision—while ignoring the mountains of evidence that might exist in the peripheral.
They become highly proficient at finding evidence that supports their ludicrous beliefs but their viewpoints are so constricted that they can’t see the forest for the trees.
Using the ladder of inference to support my point, their instilled beliefs cause them to continually select data that supports and reinforces those beliefs and that cycle only perpetuates.
Also, the more time someone invests in their beliefs and their way of viewing the world, the more traumatic it would be for them to examine evidence that proves they potentially wasted years, decades, possible the majority of their lives, chasing a pipe dream. They would have to come to grips that their numerous criticisms and denunciations of people they disagreed with were probably ill-conceived and needlessly harmful to others.
That thought is too upsetting for many people so they continue to follow the line of least resistance and reinforce their existing beliefs—doubly and triply so when someone confronts them with objectivity or worse…substantiated and observable evidence that’s contrary to what they want to be true.