At least in Florida, modern building codes of the past couple decades requires excellent insulation. The reason for so many houses being under-insulated (a sepa

At least in Florida, modern building codes of the past couple decades requires excellent insulation. The reason for so many houses being under-insulated (a separate issue from uninsulated) is because most houses are older. For the very reason you started with, houses last for many decades. It also matters how one determines under-insulated. Energy here is cheap; we use big trucks as commuter cars. Using half our electricity for heating and cooling didn't matter until CO² usage became important; now it's huge.