They already do - Residential Energy Code - 402.1.1

TLDR - R21 in the walls, R49 in the roof, R30 in the floors.

It’s paired with credits for retrofitting to a reasonable level.

Good. I've got no clue what our house insulation here in Canada is as we didn't build it. But it's way more than anywhere I lived in the US. Even at -20C or below, give us a little sun for those southern windows and we get a few degrees to keep the heat off. Although unless heat pumps work well below 0C they're only useful here as air conditioner replacements. We could have moved Moderna vaccine in the back of pickups much of the winter...

Modern air-to-air heat pumps work well in below freezing temperatures. The crossover point where the COP is 1 is something like zero Fahrenheit, but you probably don't care, since they automatically switch to resistive heating, or a hybrid mode depending on conditions and load.

At some point, as you head to colder climates, it makes sense to use a ground source heatpump instead. They're more expensive to install, but pay for themselves in areas with high heating bills.