Lumber construction is hardly the only factor. Greece commonly uses brick/stones/cement but was hit heavily by forest fires. The things in/on/surrounding those unflammable materials can still light on fire and destroy homes. On the other hand, well-prepared lumber houses often survive forest fires in America and Australia.

Unlike brick or concrete, wood is easy to make earthquake resistant, which is very relevant in a state like California. No point in building a flame resistant building that crumbles to dust within a year when it's hit by a minor earthquake.