The original concept was individual states forming a union. That union would be the reason for calling the country to be created the United States. The country would have a small central government that dealt with conflicts between the states.
The Interstate Commerce Clause was created to prevent products just passing through a state from being fully taxed. The product taxes from each state would have to apportion their tax based on the presence in the state.
This clause was to set the federal versus state powers. The Federal Govt would represent the states to the rest of the world. Whatever the Feds didn't control using the Supremacy Clause was left to the states under the 10th Amendment.
So much for history, today the concept of states much less united states is in conflict. They are today, more like the disunited states under a powerful federal government, where there is little left for the states under the 10th Amendment.
So my short answer is that it is Called USA based on historical reasons, that have eroded over time.