It's slightly more expensive, and most developers are trying to build the cheapest thing they can sell. A good number of places have put in noise separation into their building code though, so depending on where you live any new place will be dead quiet.

In a wood-frame building, for example, you increase the thickness of the unit-to-unit walls by a few inches and leave a small air-gap between two layers of insulation. The hard-soft-air-soft-hard boundary makes for a very difficult path for sound to travel through. You have to purpose-build the walls if you want maximum noise isolation, because the studs have to be staggered so they don't bridge the gap and transmit the sound through your defenses.