Because in ancient times, there WERE several different "Standard" "Feet" for measurements. In old Greek temples we see scratched into walls and floors in the work areas, standard "Roman feet" "Corinthian feet", etc. There were several standards for the foot back then and other measurements like "Cubits". The architects decided on what standard they would use and scratch it into the stonework so the workers could calibrate their rulers. ---Even back then it was known that using actual human body parts to measure was NO GOOD for precision building like they did.----But standardized measurement systems HAD been devised, they just weren't used universally---being several REGIONAL systems instead. Obviously, the FOOT was originally based on the length of a man's foot, but kingdoms had to standardize the length by the time precision buildings were being built. So we had several different standards. Construction Workers imported from other areas had to calibrate their rulers to the standard being used---so the marks on the stones determined the FOOT being used on the site.
Why aren't FEET the size of a human foot? The need for subdivisions (inches.--) INCHES were incorporated by using the lengths of common objects of the same size. A Roman Inch was the length of THREE barley seeds laid end to end. AND using 12 inches to a foot---(The ancients liked divisions of 12 as 12 can be divided into halves quarters and thirds, etc). We get the length of a ROMAN FOOT The same size as our own "ruler" "foot" today. Yes, it is bigger than a foot on the body, but, it was the closest size to a body foot, so it was called "A foot"------BTW, SHOE SIZES are still measured in terms of 1/3 of an inch, from the old barley seed measurement of the inch.