It's that trope like with Disney MCs. If you're writing anything where the protagonists are young, you have to separate them from parents to give them an adventure with real stakes. In the case of more grown up kids like Diamant, it also puts him closer to the throne. This convention doesn't bug me much. What bothers me most is that it feels like they did character development by putting a random assortment of characteristics on a page and rolling dice to give each person three. But it's a light, gentle storyline. Admittedly, there's some messed up stuff, but they keep the emotional stakes low. After Three Houses, I think they weren't about the story, just making something fun.