I read that this is because Americans see houses primarily as tax-advantaged investments, now, rather than homes. Instead of saving money (which inflates), they leverage up to the gills with a gigantic mortgage on the BIGGEST HOUSE POSSIBLE and then they both go out to work full-time to pay back the mortgage.

But then, they're both only home nights and weekends (which they spend cleaning the house, grocery shopping, and washing laundry) and they both need cars and children are a distraction, so the garden is useless, neighbors are superfluous, and the garage is gigantic.