A good example of why this is happening is in the Christmas story. Mary and Joseph were travelling via donkey in while VERY pregnant to another city to pay their taxes to the Roman government. Tax payment in those days meant putting coins or goods in the tax collector's hands. Not very convenient or practical.
The ability to use IOUs and trust as a payment vehicle solves a great many problems. Kids discover this at age 6 when they forget their lunch money. Trust is the key component, and a good reason why Bitcoin is successful. (example: The Chinese are always exploiting any means possible to get money in forms other than RMB)