Because 1 foot equals 12 inches, 5 feet equals 60 inches. Feet and inches are U.S. customary units of length derived from the Imperial system of measurement.
Before there were instruments of measurement, parts of the human body and certain objects were used for measurement purposes. Around 1150, according to Reference.com, the inch was equal to "the breadth of a man's thumb." When Edward II was King of England, the inch became “three grains of barley, dry and round, placed end to end lengthwise.” With the conquest of the Normans in 1066, England adopted the Roman definition of a 12-inch foot. The inch and foot became part of the imperial system and are now used primarily in the United States.