Quick Google says a great majority of Americans take road trips. Even though it's a tiny fraction of their driving, it's still a deciding factor for many when choosing a car. Not all people have the luxury of affording a second car just for road trips.
Public transportation would be good, but there's less flexibility to it. For example, just yesterday, on a return from a roadtrip, I got stomach sick and had to request frequent stops. That wouldn't fly on a train.
I'd love it if we had affordable and flexible public transport for getting all across the country, though.