With all the technological advances, why has fuel economy remained at 21 mpg for the past decade for autos?

Answer 1

I don't know where you got the information that fuel economy is at 21 mpg but let's assume that is correct.
This is a question of supply and demand. If people were more interested in the fuel consumption of their car then other things, car manufacturers would build their cars with good fuel economy as their primary goal. If a safe car is what people want, a safe car is what the manufacturers would build.

Answer 2

easier to sell power than it is to sell fuel economy...unfortunate, but it's true

Answer 3

American housewives are in an "arms race" to buy the safest SUV.