Why so thin 
Because its just to spread out the load, note I followed with "filled with dirt"
As in the ENTIRE above-ground house is basically one big dirt filled block, on top of a thick slab, with the dirt taken from underneath. Basically, it looks like a house, but its actually the overburden for an underground space. Kinda a stealth bunker. Basically, big ranch style home, built on top of a "overly thick" reinforced slab with "overly deep" footings/basement walls and a concrete floor basement...but with the dirt removed after the fact and simply filling the above-ground box.
So there is nothing to ram into (other than the passage-way). So if you bust door, you hit another door...so they bring out the ram-equipped MRAP and drive it...into a reinforced concrete block.
Splat goes the MRAP.
Hell, drive an M1 into it. The tank would go from 20-0 in about 6". 60 ton tank vs 500 tons of dirt.
