In New Zealand our sound insulation requirements mean that every shared tenancy wall I've ever had has been almost impossible to hear the person next door. It doesn't seem to be prohibitively expensive to do, although it certainly helps that it's a regulation from the city councils rather than up to the developer.
I think you are right: A city government or business wouldn't do this because they would have to take on the entire cost themselves because they don't have any control over the other end of the more global balance sheet: the rural and suburban areas.
A state-level entity could mandate it as part of construction code for 2 reasons:
1) Even the higher construction-cost apartments would be more financially solvent over the long-run than continuing to subsidize the alternative rural or suburban lifestyles.
2) Having nicer attractive apartment lifestyles would entice more people to move to more sustainable urban areas rather than the environmentally and economically unsustainable rural and suburban areas.
Why would a commercial (or government) development project invest in something unprofitable (or unnecessary to their requirements)?