I guess that's true about all lit forms, isn't it? That the characters who share our assumptions about what life is supposed to be like are the ones we're meant to care about, and the ones who have differing assumptions are only there in the story as accents to the overall decor?

It's a shortcut, I guess. The more formulaic the story, the more heavily it will lean on shortcuts. Romance, as a genre, is strongly formulaic, so maybe that's why we see this shortcut more consistently in this genre than in others?

Which raises the question: how do romance authors know their readers will share those particular assumptions? Probably a chicken-and-egg thing: the romance audience is self-selected for those who want the particular conventions which the romance genre supplies.