The Vanishing Hitchhiker is a defining urban legend, and not specific to the United States, though the below reference by Jan Harold Brunvand is primarily about American urban legends.
Why the story is told is a good question. It is an encounter with the supernatural — a ghost is met, and the driver was not looking for the supernatural; it came to him/her. The spirit world may be met at any time, by anyone. The spectre passes for a living person. The driver does not recognize it as a ghost at the time, and we never know when we meet the supernatural.
All in all, it's a great story to tell kids, preferably late at night, over a low fire.