you are not investing in something
That's, pedantically, true. If don't create capital with it, "investing into Bitcoin" may sound grammatically incorrect. Neither do you, though, when you use a fiat currency. Don't mess investments with currencies, let's take it one at a time.
Since I've seen you're keen on definitions, let's see how Wikipedia defines what's "currency":
A currency[a] in the most specific sense is money in any form when in use or circulation as a medium of exchange, especially circulating banknotes and coins.[1][2] A more general definition is that a currency is a system of money (monetary units) in common use, especially for people in a nation.[3] Under this definition, U.S. dollars (US$), euros (€), Indian rupee (₹), Japanese yen (¥), and pounds sterling (£) are examples of currencies.
So, a currency is, by definition, used as intermediary to exchanging. A country may create its own to handle the economy. It, therefore, legalizes its use. People use the currency, because they live inside the country. The administration of it happens from the central bank of the country.
Everything's good so far?