In California, a big difference is earthquake retrofitting. Older houses often aren't retrofit until sale, and retrofitting is expensive.

Also, I've noticed newer houses (2000s-present) are airier and have more natural light compared to older (pre-90s) houses.

> Also, I've noticed newer houses (2000s-present) are airier and have more natural light compared to older (pre-90s) houses.

This depends a bit more on the specific era of the home, rather than just being pre-90s, but it's particularly present in 1970s housing stock. The energy crisis led to a style of home design that favored small windows (energy efficient windows weren't yet really a thing, or at least not widely available at affordable prices) with lower ceilings. After the energy crisis through the 80s things started opening up again, and then improvements in insulation and windows (for two specific features) and improved efficiency in AC units led to a further opening up of home designs (with larger windows, higher ceilings) through the 90s and to today.

Newer CA homes also tend to do better in fires.