"Expensive" is in the eye of the payer. Do you think a few thousand is expensive? A developer absolutely does. Also, it takes more space, so doing it after the fact may lower your ceiling more than you're comfortable with (assuming your landlord is even willing.)

We could solve this with code updates and strict enforcement. However, pro business types will complain that it increases rents (probably true in the short term) and drives down development (probably not actually true.) Also, better sound management generates more livable spaces that better support city growth in 10 or 20 years, which is also important from a sustainability point of view. But nobody wants to pay for that. That's "someone else's" problem.