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.