Because they can print as much money as they want, devaluing yours in the process

The observation that is politically relevant is that availability of money is not a constraint for the capacity of the state to ensure adequate provisions for everyone, as long as such provisions continue to be available through production.

Cryptocurrency is based on a fantasy of apolitical money. Bitcoin is not currency because designation of an asset as currency is political. The moment everyone realizes that Bitcoin has neither intrinsic value, like gold, nor state backing, like the dollar, the value crashes.