There will always be people that claim something in the face of overwhelming evidence to the contrary. It's not a rational position, so it quickly becomes very complicated to figure out why someone would keep holding on to it. It can be (and frequently is) different for each person that holds an identical position so it is also not a super useful thing to figure out. Unless you are trying to unpack why a loved one holds an irrational position, of course. But the venue for that is maybe not HN :)