Cryptocurrency is decentralized and not under their control. If they will use it it will only mean that they acknolwdge crypto as legally accepted currency which they have bo jurisdiction. Who don't even know if the government is not really using crypto or if they have been using it secretly.
I want to say that the government who are always on the look for every opportunity to invest and make it fast can not claim or out rightly deny the fact about the knowledge of Crypto currency. And also possible investment on a secret note, but the Volatility of bitcoin which every investor know and is aware of could possibly be one of the strong reasons why the world government might be scared of using crypto currency. Also most of the government actually want control over the cash inflow and management. In a case where they would not be able to control and determine the price control of their money in use can actually cause reluctance on the part of the government on adopting crypto currency as a currency for use world wide.