Oil companies won't allow it. It will cause them to lose all of their business. That's why standards are so low. Cars are capable of getting 50 mpg but companies only make them so they can achieve 28mpg and then tell the public that's amazing.
I read somewhere that they had done a test in Arizona or something. They made 100 either electric cars or extremely fuel efficient cars. They had these families test them for 3 months. They were great, the families loved them but the cars were taken away after the test period even though the families requested to keep them and then the whole study was never heard about again.
The greed of oil companies is why we don't have good electric cars yet. Not to mention the fact that electric car commercials sabotage themselves by making fun of electric car owners having to stop at gas stations.