Most people don't have houses large enough to house multiple generations inside the house. Houses are sized for parents + kids. And those are the only dwelling units available or legally allowed for vast distances in any direction.
From what I've seen American bedrooms are far larger that the 12 square meter bedrooms that are normal for two or three children to share in my country. Grandparents don't need a separate wing, they need a room.
Too monotonous housing mixes over too large of areas.