LA County is a little over 4000 sq miles and most if it is arid including the mountainsides at lower elevations.
The undergrowth you refer to is commonly known as chaparral and is what prevents those mountainsides from coming down when it does rain and it does at times.
The east winds, no matter what they’re referred to are hot dry winds blowing anywhere from 20 to 60 MPH and when fires start will not be controlled until those winds subside or the fires run out of fuel to burn.
The current fires in northern CA are due to these high, dry winds. They can’t landscape thousands of square miles in CA and as the population grows those people have to live somewhere.
That’s the way it’s been and will continue to be.
"The Control of Nature" by John McPhee offers some good perspective on this.