When was the last time anyone hauled coal on the interstate highway system? And is it really practical to move a 400-car load of wheat in highway trucks? For any really voluminous and heavy cargo, if you are not near a major body of water, there is only rail.
For long hauls you have a point, but remember the bulk cargo is (almost) always hauled to a collection point for long haul by truck. Also normally air freight is only used for urgent high value cargos, but was used in the Berlin airlift to haul coal.
Stripminers in Ohio and West Virginia regularly run coal trucks on Interstate highways ~ not all mines are accessible by rail.