The issue isn't the big one. The issue is a minor tremor that happens to crack a vital support beam, or cracks a pipe, and causes $25k of damage.
Making the insurance executives ultra-rich.
Insurance agencies serve primarily as vendors, customer service, etc., but the risk is carried by the CEA. The remaining 1/3 of policies are carried by a few private insurers who still underwrite policies.
In theory, that system should prevent insolvency. Remember that California is a huge state and even a very strong earthquake would still have fairly localized damage. For example, the 1906 earthquake (Richter 8.0) that levelled San Francisco did comparatively little damage to Petaluma, a city 40 miles to the north.
A $250k bill is a different matter, that’s the sort of thing that will bankrupt many.