Unfortunately insulating a building after the fact is extremely difficult, expensive, and depending on climate can approach the level of near tear down. I think it many cases it may ultimately be more environmentally friendly and cost effective to add things like solar and just brute force the heating/cooling.

The reason for this is that buildings not designed for insulation and air sealing can suffer from dramatically worse durability (mold etc) when just adding insulation. It is not necessarily wise to just add fiberglass batts to wall cavities.

The addition of insulation to our homes is a modern thing, and there have been significant issues even currently (the mold crisis).

The issue is that to build a building properly, it is necessary to address heat flow, vapor flow, air flow, condensation, drying, and water barriers. This means you often have to address the entire envelope of the building, which is very expensive, difficult, and means replacing a lot of material. Basically remove all cladding and sheathing and upgrade it.

Realistically most of these concepts are not well understood in the construction industry, and the expense is high so most corners would be cut leading to huge issues with mold and wood destroying pests down the road.