Because biology tells them to.
Believe it or not we weren’t always civilized and we didn’t always have morals. Humanity has been around for perhaps a million years, and we’ve only had societies to tell us we shouldn’t be attracted to children for perhaps the last 3000 of them.
Nature gives exactly zero fucks about your morals. As far as it’s concerned, men being attracted to 12 year old girls is exactly how it wanted it to be.
You can’t reverse hundreds of thousands of years of biological conditioning with a few of just telling us it’s wrong. On one level it’s a psychological phenomenon, on the other it’s a resource problem. To rise above our baser natures, to build civilization and civilized people, requires just massive amounts of work.