its like getting your parents blessings basically
Because they are minors
Asian cultures are collectivist in nature as opposed to western cultures which are individualistic. In Asian cultures the family unit is prioritized and the parents (father/mother/grandparents) are extremely respected and honored. This respect of the elders is socially/psychologically ingrained, and so, eventhough legally, parental approval is not needed, from a social moral standpoint their approval and blessing is absolutely required.
and why would they need their parent's approval? They decide for themselves who they want to be with for god sake.