I think they just believe it when people give them empty compliments. Girls hype each other up and baby each other if they look even slightly better than usual, and guys tell them they're pretty when they're trying to slide into their DMs... It seems like they start believing their full-makeup, filtered face is their actual face. I realized this after seeing so many average-looking girls around me who think they're gorgeous. Then those girls get 'appearance dysmorphia' because they don't look as good as they thought when they see themselves in the mirror or the back camera.

For the record, I'm writing this because I used to be like that too.