Because the resource price itself is relatively low (the most expensive material is gold, but it's not in used in big quantities). Silicium is quite cheap.and makes up the main part. And that's pretty much the reason, materials take only 5% of the price, the rest is in die making, cooler production and the PCB manufacturing. While why they make only extremely powerful parts? It's for finantial reasons, not everybody can grow money in theur garden and most of the population finds cards like GTX950 and R9 270X far more than enough.
Also, let's not even bother thinking about quadro and fire pro gpus which are essentially just a bit more powerful than their consumer counterparts while being 10 times more expensive. Their real reason of existance lyes in their error fixing abilities.