Is the world becoming uninsurable?

Americans used to build cities with brick and masonry. They were repeatedly destroyed by strong earthquakes, as would happen to your city if subject to similarly severe earthquakes. Americans paid for that lesson in blood.

European houses are not designed to withstand American disasters. A brick house that can survive a M8.5 earthquake, which is the safety standard where I live, will be almost purely steel structurally and very expensive to build. The brick would be decorative, which can be (and is) done on a wood frame.

I definitely understand what you are saying here, and it makes sense. But concrete is quite common in Europe these days, which I suspect would also be a good option for earthquake zones.

The entire south and south-east of Europe has a similar seismic risk to most of California, and wooden houses are nowhere to be seen.

Concrete + rebar and then a steel roof secured with hurricane-proof metal straps, or just tile roofing if the area isn’t hurricane prone. Concrete can also be used for things like insulated concrete forms (ICF) that save energy and improve insulation for both hot and cold.

In LA a lot of non-mansions were destroyed but plenty of them were modest to reduce overall cost initially because the area has always been expensive to build or live in, even for the original homes to be put up.

Then you have to consider how quickly development took place by comparison, and the collective degree of certainty among the original buyers on whether or not they would be able to afford to stay very long anyway.

So many come there just to give California a try since it's supposed to be the golden state, who are depending completely on the occurrence of good fortune within a limited amount of time before they would expect to return to states with less-expensive hometowns in mostly less fire-prone environments.

This would influence what kind of home they would expect to be suitable for their needs to begin with, and how long it might need to endure.