The amount of energy needed to completely burn down a tree is higher than the amount of energy to burn down a house.
The fuel to feed a fire around a tree is likely just going to be the surrounding vegetation, so lawn grass. The grass catches quickly, burns up, tries out the nearby grass, the fire moves to the new dry spot and dies out in the previous spot.
A house is just full of fuel and surface area for a fire to catch.
The amount of energy needed to completely burn down a tree is higher than the amount of energy to burn down a house.
The fuel to feed a fire around a tree is likely just going to be the surrounding vegetation, so lawn grass. The grass catches quickly, burns up, tries out the nearby grass, the fire moves to the new dry spot and dies out in the previous spot.
A house is just full of fuel and surface area for a fire to catch.
Another way to express this is that bark is more fireproof than vinyl siding.
The density of houses would also contribute significantly here.