. But they still don’t exist in a vacuum, and a lot of public money goes into the network that links the Turnpikes to the destinations.
Exactly. For the better part of a century now the United States has used public money (city streets, free parking), tax expenditures (tax deductible mortgage interest), mortgage guarantees, zoning and other elements of public policy to create a country in which most people need a car to live at all conveniently, and many people need a car to be able to work and shop at all. Private investors have done the same, from mall and subdivision developers to companies choosing to locate where employees more or less have to drive.
Next to that, all the state, local, and federal transit, bike paths, intercity rail, and intercity bus subsidies are a pretty tiny pile of money and resources.
Once people own a car, it is much harder to get them to pay the full cost of any other mode of transport. Once nearly everyone owns a car, it become politically difficult to tax gas enough to discourage driving (and the pollution and huge highways, parking lots, etc.) that go with it, or even to cover the cost of the road system. Once people know there is a gas tax, they miss the fact that a lot of general revenue also goes to streets and other car infrastructure. When people in the US hear that Europe, Japan, and some other places tax gas and cars at very high levels as luxuries and provide really frequent public transportation all over, many of them cry out socialism, because they don't understand that government policy shapes and subsidizes the car-based society they live in.