Why doesn't Yua call his parents Mom or dad? or papa/father.

someone already discussed this on an earlier comment but basically it's because they never influenced him to call them mother/father and I think it would be weird if he started calling someone mother when there's nobody resembeling that figure, tbh it's way better this way just like how some parents teach their kids to call them certain ways kokone and nagi just didn't and he just calls them by their pref3red names