They don't have to. The apt building I live in must be built really well because I have never heard a single sound from the people around me. I can only just hear muffled talking/music while standing in the hallway but never from within my own apt.
Do you have a sense on how bad the increased cost is for reasonable sound isolation (talking/tv not garage band)? Because I have dealt with loud neighbors, and if the problem could be ameliorated for an extra $1/m or something else trivial, I will be enraged that it is not mandated everywhere.
