Well.... public transportation requires infrastructure which is not always around. In the south-east USA, for instance, there are public buses here and there but they rarely go for more than a few miles, and there are no trains or subways to travel mid-distance or between cities.
So grey-hound or car rentals are your best bet unless you already own a car.
So grey-hound or car rentals are your best bet unless you already own a car.