It's because the common thought of most koreans or anyone(which I only observed through manhwas and kdramas which I don't really think credible) is that orphans have no parents and parents have the vital role in a person's life so they kinda think orphans have no manners or act like savages coz they don't have parents and the prejudice that because no one provides for them they are always treated like thieves
So as somebody who was orphaned until I was 5, kids truly don’t understand the concept of adoption and the lack of control a child has over that. Because they aren’t self aware and mature at that age, children will resort to petty teasing and bullying. Simply because they want to feel better about themselves.
I don't know why some people are so prejudiced against orphans. As far as I know, having parents doesn't mean you're way better than those without parents. Some parents don't deserve to have children, even.