Because I am in the construction business, and while I don't build new homes I get to tear apart lots of older homes. And in homes built prior to WWII you often see no insulation at all in homes, but by the time you get to the late 1970s everything is insulated up the wazoo. Homeowners don't like to spend $600/month to heat a home whereas it would cost $150 if it were insulated. They also don't like to sweat in the summer because the A/C can't keep up with the heat gain.
Who are these people who would buy a new home that is poorly insulated? People buy new homes in part because they're well insulated, unlike older homes which often have no insulation at all.
No, before A/C was common windows were carefully placed to ensure air flow through the house. Insulation does not help at all if you don't have A/C, in fact it can make it much worse as it locks in heat from hot water heaters, ovens, stoves, televisions, light bulbs, etc.
Cooling and heating both factor in the equation. In which case you have to look at the whole picture, from summer highs to winter lows and weigh that against the typical 72-75oF comfort zone. The further away you are from that on any given day the more energy you will use for heating/cooling.
I concentrated on cooling in Texas because I'd guess Texans use far more energy cooling than they do heating.