All it takes is the tiniest spark to start a fire. A dragging hitch chain is enough to do it.
The building density is high but the brush and other flammable plants are between the buildings from the top of the mountains down to the ocean. The tinder was set long ago, just waiting for the ideal conditions. Everyone knew it.
In the past few yesrs, it has not been possible (or feasible) to insure many properties against fire. Many people were dropped by their insurance companies and insurance companies left the California market entirely. That means that many of the properties which burned are a total loss.
The fire will make it even more difficult/expensive to insure against loss. The insurance companies will pass the bill on to those of us who still have insurance to pay.