It’s a very dark part of conspiracism in general. The same tactics, both conscious and unconscious, are used to evangelise these ideas - and defend them despite being indefensible - as are used in all conspiracy theories and “alternative” views of established fact.

So, it has less to do with the available evidence, and more to do with personality flaws. It’s not even about reasoning skills or intelligence - the more intelligent you are, the less likely you’ll be to change your views because you’re so good at generating narratives that support your position. It’s a deep flaw in human psychology that can’t be reasoned away, and trying to combat these ideas with facts just reinforces them and gives them credibility (which is why no one with any sense debates Holocaust deniers anymore). It’s like when a schizophrenic person hallucinates; you don’t want to do or say anything that makes the hallucination seem real, you don’t want to say “where is the creature? Here? I’m stamping on it, is it gone? I don’t see it!” you simply accept that they’re hallucinating and don’t engage with it beyond that. Extreme example, but the logic is the same.