Why do parents...?

Very true that hmmmmm

You'd have to ask the specific parents. I don't expect my children to refer to me as "Miss" my last name, nor do they answer their father with, "Mr."

Which Country?

Because in most civilized countries we feel that the relationship between parents and children should NOT be as formal as for between teachers and children. My child doesn't need to call me "sir" in order for me to feel respected. You appear to be living in a very conservative, rigid, primitive sort of place.