Also American. East coast. I've been in tons of houses with separate dining rooms, as well as living / family rooms.

And yeah, nobody used em. A friend of mine growing up had both living and family room in his house. One was used all the time. The other was untouched, pristine. Like they might have even had plastic covering the furniture. We weren't supposed to play there; it was only for formal entertaining.

My in laws also have a separate living room with a piano in it. And besides the off chance someone tried playing chopsticks, the room is unused. Infact, most of us put our coats there. The family room has a TV in it, so everyone goes in there.

Heck, the house I grew up in had both a kitchen and dining room. I think the dining room only got used on holidays when we had guests over.

Not sure why you haven't seen as much. Might have to do with the size of the house, demographics, or region.

But boy, are they stupid. Rooms everyone thinks they need, but nobody uses.