US soundproofing code has always been lacking because code was mostly invented to solidify single-family detached housing. For example, the 2019 California Residential Code doesn't actually mandate anything related to soundproofing, but includes an _optional_ section on soundproofing in Appendix K[1], which mandates walls and ceilings have an STC of 45, about the same insulating capabilities as a 4" brick face wall [2]. Some cities have opted into these requirements. Notably if you live in the SF Bay Area, neither San Francisco nor Berkeley mandate any soundproofing (e.g. they do _not_ mandate adherence to Appendix K), but the City of Oakland _does_. This is the real reason shared dwellings are so noisy; code doesn't care.
[1]: https://codes.iccsafe.org/content/CRC2019P1/appendix-k-sound...
[2]: https://www.hud.gov/sites/documents/DOC_16419.PDF
