Because people won't buy anything else - and why should they?! I mean (at least in Europe), the cost of the actual building materials is dwarfed by the cost of

Because people won't buy anything else - and why should they?! I mean (at least in Europe), the cost of the actual building materials is dwarfed by the cost of the plot on which the house is built, the interior fittings and above all the labor costs of building the house. So going cheap on the building materials is simply not worth it...