If the developer doesn’t want to add one, that’s perfectly fine.
However, I’d just like to say that I don’t really see “it doesn’t make sense with the characters” as an argument. If we accept that concubinage is a thing in this kingdom, then there would be no legal issues or “scandals” with the king having a proper wife and a few concubines on top of that. That seems to be Lilith and Selena’s only issue, since they know you’re sleeping around, so it wouldn’t seem that far-fetched for them to let you stay with the other girls. Aside form that, Evelyn and Callie both seem open to sharing you, and Samarra is already of the opinon that you can have whatever you can take for yourself—and as king you’re powerful enough to provide for all of them. Nell might be reluctant at first, but Samarra could help change her mind somehow. Primrose is just about the only character for who I have a hard time imagining a way to make her OK with this kind of situtation.
I’m not trying to complain, here; I just want to say my piece. Again, if the dev doesn’t want to make a harem ending, that’s the end of it. There doesn’t have to be a reason. But I don’t think “it doesn’t fit the characters” is true in this game.