Because then car companies have to maintain two systems and the complexity increases. I'm very happy with my plug-in hybrid. I'm running 100% electric in my day-to-day driving and on longer trips the amount of gas used varies. If I'm careful then I can go a year without a trip to the gas station. I wish more companies went this route, but they want to keep their profit margins up. I'm just waiting for the flood of cheap EVs that will eventually come in when more and more companies enter the market. Chinese EV companies are looking to enter other markets and this should drive prices down, but legislators are proposing tariffs to keep prices up.