I think good sound isolation mostly comes as a side effect of concrete housing, or things like firewalls in the american housing market. I think it's a side effect of how house price = sqft so all the other important things go by the wayside.

When I looked up soundproofing, there are techniques that involve a few layers of drywall and sacrificing like a few inches of air space to have an airgap, so it is possible to do it relatively cheaply in new construction, but then you sacrifice sqft :/