You might as well ask why the Carolingian Empire (and most of Europe for several more centuries) used librae in its accounts or why tribute to Athens was stated in talents, neither of which was ever coined. Uncoined units of account are a convenience that avoids the need of coins for monetary transactions and that cannot readily be debased. See Dsvid Graeber's Debt: The First 5000 Years for an extensive discussion. Units of account that do not correspond to coins are a normal feature of human civilizations, not something requiring a special explanation.
C Monsour
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