US houses and apartments seem notoriously poorly insulated when it comes to sound. As you say, its a small extra cost, and it can allow for more people in an area (even having connected townhouses or apartments) without having to deal with neighbor noises. If you can fit 3x the number of houses into a lot and they have good sound insulation, you're producing a lot more revenue from the house sales as well as generating potentially much tax revenue for that area of land (depending on how property taxing is done there)
US houses and apartments seem notoriously poorly insulated when it comes to sound. As you say, its a small extra cost, and it can allow for more people in an area (even having connected townhouses or apartments) without having to deal with neighbor noises. If you can fit 3x the number of houses into a lot and they have good sound insulation, you're producing a lot more revenue from the house sales as well as generating potentially much tax revenue for that area of land (depending on how property taxing is done there)