This is an interesting question. I went down a rabbit hole and found a good article about it https://theconversation.com/sirven-para-algo-los-cortafuegos...

Tldr: Fire stoppers (or whatever they are called in English) don't really work that well anymore, at least in Spain. This is due to the increasing amount of combustible matter in the forest.

Also, Spain is prone to flash floods, so having a patch of mountain without any trees can quickly convert it into a mud avalanche or make the flood worse

Also strong winds, which allow the fire embers to "jump" rather long distances. This is exactly what's happening with the Madrid wildfire.