The city of Silent Creek was a Nascent Level city, with its City Leader being at the Nascent Soul Realm. Different cities have different power structures, the City Leader for the Silent Creek City didn’t want to create a royal family and rule over the city.
He just wanted watch to over the city as a City Leader, and when he was tired of it he’ll pass it on to someone he deemed worthy. Though that person would have to be of the Nascent Soul Realm.
This type of power structure wasn’t uncommon. There were various cultivators in the higher realms who instead of going into seclusion would go out and create a city for themselves. So there were many cities like these on the Maple Continent.
As for Cities with a royal family as the leader, those were few in number. And one should note that this does not mean their above the Emperor obviously. As even kings and queens needed to bow before an Emperor.