The definition of a fork is centrally regulated, yes, but
using forks is decentralized. There is no central authority saying "We will make N forks this month" or "You will use a fork at 2pm for exactly 10 bites".
Cash is similarly decentralized in that its definition and creation is centrally regulated, but nobody says (or even can say) how and when you can use it. Cash transactions between people happen without any central authority. There's not even a central authority definiting the price/value of goods! You can just agree on a number with some rando, exchange cash for things/services, and nobody will ever know it even happened, let alone be able to tell you how and when to do it.