This can't be it, much of the housing stock is from pre 1950s and adding those finishes or code improvements does not make a house 5x as valuable. My theory is

This can't be it, much of the housing stock is from pre 1950s and adding those finishes or code improvements does not make a house 5x as valuable. My theory is that we simply don't allow building anywhere near the rate of increasing demand, which is reflected by the anemic supply growth compared to population increases.