See this picture? A car made of metal melted and a house burned down. But yet trees made of wood are still alive? Some would have you believe this is the result of DEW, Directed Energy Weapons (basically meaning lasers) being used to start fires. Some even claim Trump is behind it because he hates California liberals.
The truth is pretty simple though. Trees are 50% or more water, and they are constantly absorbing more water from the ground. Cars on the other hand are made of metal, which retains heat, plastic and rubber which burn hot and long, and interior fabric/padding which as anyone who ever burned a couch knows is better than kindling. Cars also typically contain gallons of literal fuel.
Houses though, they are kinda funny. In that they are made of wood like trees are, right? Wrong. House wood is dead, dry, and not absorbing water(through a root system,) besides that houses are covered by often flammable siding and roofing. But not always. Some homes, especially newer or more properly built (and likely more expensive) homes, have fire resistant siding and roofing, adding to the explanation of why some houses burn, and others don't. Add to the the different designs of homes, some having unprotected wooden decks, some having good airflow to fan the flames, others sitting on fuel lines or tanks which can rupture and explode, and all of them full of flammable ass furniture.
So to recap, trees are filled with water, cars are basically a pyre, and houses vary from one to the other, but contain enough fuel to keep a camp fire burning for a month straight.
Now if only fires had a natural way of spreading, like combustion from heat or flying embers...