When millions of Americans live in rural or suburban areas, what do you think is easier? Extending rail and bus routes to within walking distance of every house, or convincing everyone to move to a city? The answer is neither, and that's why cars exist.
Here's one that isn't a trick question: is it more efficient to build a rail line to my office and have a train run there or to have individual cars drive there? I don't know the answer for sure, but considering you may possibly need 15 trains filled mainly with people who don't work there converging on one location or 15 cars with one person each who does work there converging on one location, I'm almost willing to bet 15 electric cars would be more efficient than 15 trains.
There's a certain density requirement that mass transit needs that doesn't always exist. That's why we have cars.