No one really talks about it but I think it is a symptom of bad parenting. How many times have people sat their kid down and explained these Hollywood types are not the average looking person?
Do you have children? How old are they? Do you actually believe that "sitting your kid down and explaining XXX" to them is likely to cause substantive changes in what are fundamentally deeply non-cognitive takes on the world?
I'm pretty sure the people suffering are well aware of that. This sort of mental illness isn't the kind of thing thats rooted in logic-brain, and simply "explaining" it doesn't make it go away, else society wouldn't have as much of a problem.
Today, you also have to explain that even the pictures of your friends are not "the average looking person" because almost everything online gets fed through various filters.