That's not correct. California's housing stock falls into something closer to the US median. The oldest housing stock in the US is in the North East and Midwest (which tends to be well insulated for Winter purposes). Those regions saw large waves of high population expansion and housing development previously (50+ years ago), which put down an unsurprisingly large base of housing stock.

California's development by contrast has overwhelmingly occurred in the last 50-60 years (their population has increased four fold since 1950, from ~10m to ~40m today). Compare that to Ohio, which has seen a nearly flat population for coming up on 50 years. New York State's population has only slightly increased in 50 years.

The newest housing stock is in the Idaho/Nevada/Arizona region, Texas and the South East (Florida to North Carolina).

The US generally - and much of California - also aggressively uses air conditioning (central or otherwise) and has for a long time, which has further spurred efforts toward maximizing insulation.