Why do cars still not tell you how much battery life is left when the engine isn't running?

Does anybody else think that would be nice to have? If you are, say playing music whilst sitting in your car, but don't want to leave it idle to waste gas, wouldn't you like to know how long before you need to start the car to charge the battery? Seriously, it's 2011, and nearly every other device with a rechargeable battery has a battery indicator, yet I've never seen a car that does this. Has anybody seen this in any cars? If not, why do cars have yet to indicate battery life?