There are psychological effects at play in these things.
Notably confirmation bias where people tend to read and listen to things which confirm their view of things and ignore things which don’t. And cognitive dissonance where people, when presented with evidence which shows them to be wrong, simply dig their heels in, dismiss the evidence and believe themselves to be right all the more. The more invested people are in an idea, the stronger this effect is and the harder it is for them to admit they were wrong.
I would recommend Black Box Thinking by Matthew Syed which talks about these things.
I’d have to admit Leavers don’t have a monopoly on this but what really annoys me is when Johnson et al have previously said no deal would be a bad thing, now they’re very upbeat about it and no one seems to be picking up on it and all the other glaring contradictions in the things they’ve said over the years