Earlier quoted context omitted.
You have a very incorrect build of what modern American construction is like. Yes it is mostly timer-framed, but we have fire codes which means walls are not as thin as you seem to think. Sure if you look at an apartment built in 1950 your summary is valid, but that isn't done anymore. 5 over 1 refers to section 1 and section 5 of the building code and not the number of floors built for each. It is coincidence that w…
it's still cheap and shitty and doesn't isolate sound, causing all sorts of problems with the neighbors that don't need to exist. thick concrete walls might as well be hypothetical, theoretical building material given how much not expensive it is, and thus how much rarer it is.
The use of masonry is limited by the requirements to withstand severe earthquakes, which are endemic throughout much of the US. I’ve lived in apartments that were effectively soundproof but those were legacy buildings where the interior concrete and masonry walls were not engineered to withstand earthquakes. It is pretty expensive to build apartments that are soundproof and can survive a magnitude 8+ earthquake. You can’t just throw building mass at the problem as is so common in Europe. Buildings need to have a lot of elastic flex to them.