Anecdotal example: I'm currently living above a cafe, with an entire floor (with an office in it) between us and them.
When they drag the tables and chairs around, we can hear it easily, because it sets up a vibration through the structure of the building. Took us a long time to realise where the noise came from — it sounds like it comes from above, but the flat above us has been unoccupied on some of the occasions we heard the noise, and we were only able to confirm the real origin by me being outside the cafe watching them while my partner was inside, and with the two of us on a call so we could directly observe that what the cafe staff were doing correlated with the noises he was hearing.
Both, increasingly the later, in a mix of cost (and who shoulders it and when), regulation and in how far we design around our shortcomings.
For example, a window is a very obvious thing. Obvious things, we value. A balcony. A garden. These things either exist in some capacity or they do not, and it's easily to mentally check them.
Acoustic treatment is more akin to differences in heat insulation. You notice it, when it's a problem, at best. Or it's a problem that you pay for all the time, but you don't even know how much. If buyers/renters don't value something directly, even if it impacts the negatively, the incentives on the builders end to spent on it are understandably low. It requires regulation to be done properly (which a lot of countries have understood and, hence, regulated).
Both I'd say. We consider it less than the thermal problem since there is less regulations about noise, and it's also part of how we build today since there is a tradeoff between price, sound isolation and even thermal isolation. But we do have good techniques.
It got real expensive here to build multifamily housing in part because you have to put acoustic separation in shared walls now. That and a tight construction market are putting the brakes on what would otherwise be unwise levels of growth. And therefore unlikely to change except by accident.
