Why would they do that when Leia was not even born yet? The office of QUEEN of Naboo is an elected position. Padme Amidala was no longer queen when Leia was born, she was a Senator. Padme called her successor "Queen" so it was not just a courtesy title for Padme it was the title of the occupant of her office.
Also, Vader called Leia Your Highness in Episode IV, a title of royalty. This was when he didn't even know about the twins, Luke and Leia, thus making Leia his daughter. As far as he knew, she was the daughter of Senator Baal Organa of Alderaan.
Like I said, a Star Wars "thing". :-)