As others have pointed out already - there are cars/technologies out there that can exceed that limit; however, various amounts of money would need to be invested to either market or expand on current technologies. Plus the mindset is "we're doing what we've always done and it works" so why change? Not saying it's right just oil companies control a lot!
When gas prices raise people think about smaller more efficient cars, gas goes down people think about big trucks and performance cars and back and forth it goes. I have a diesel truck and that is one of the biggest scams out there right now and in previous years - diesel is more expensive than gas; for that matter diesel is more expensive than kerosene. It is absolutely ridiculous!