They have. But drag is drag and the price of fuel is the price of fuel. Drag increasing at higher speed means your fuel usage grows geometrically with speed. Slower is more efficient (to an extent, slower means higher angle-of-attack to maintain lift so your drag will start going up again if you get slow enough). No matter how efficient the engines get drag remains the same and going faster will still cost more. Engine efficiency just goes into reducing operating costs instead of travel times now.