Because that's how money is defined. At least, the first few hits on Google for "money definition" are:

1) <Google> "a current medium of exchange in the form of coins and banknotes; coins and banknotes collectively."

2) <Merriam-Webster> "something generally accepted as a medium of exchange, a measure of value, or a means of payment: as a : officially coined or stamped metal currency b : money of account c : paper money"

3) <Investopedia> "An officially-issued legal tender generally consisting of currency and coin. [...]"