There's another aspect to firestorms which people tend to forget.
Even if you build the most fireproof structure possible, with a shelter to cover from the flame front, an intense firestorm can kill by taking all available oxygen out of the air - and it only needs to so do in any particular area for 5-6 minutes whilst it's passing over.
The ecology of many US forests is based around periodic fires keeping the undercanopy relatively clear and many trees have evolved bark tough enough to handle minor fires (as with many Australian trees, some American trees require fire to germinate too) The US Forest Service only realised the mistake of their "stop all fires" campaigns in the late 1980s - by which point decades of misguided human intervention had left megatonnes of flammables piled up, making the fires that did break out and get away on firefighters that much more deadly. That's still being addressed. Couple that with increasing urban expansion into vulnerable areas (and the factor that some californian plants may as well be producing creosote or turpentine - particularly Ericameria laricifolia, but also the imported eucalyptus varieties) and you have a recipe for disaster, but it's impossible to prevent people living where they want to in a "free country"
Yes, people usually know the risks where they're living, but when things happen they frequently happen so fast there's no time to react - and with the wrong conditions a fire can easily jump a couple of miles (especially if there are eucalypts involved and California has a lot of those in some areas)
It's a bit like earthquakes. You learn to live with the danger and to design for it, but that doesn't lessen the shock or grief when it happens.