Cash is a specific form of money. There is no such thing as digital cash (or maybe, all digital money is "digital cash"). Digital money on the other hand is all around us. I can't remember the last time I paid for something with cash. But I spend digital money every day. I get paid with digital money each month, I sit on the couch and spend digital money on Amazon, and I routinely tap my iPhone to use Apple Pay to spend digital money in shops. It's just moving bits around a database.
Reading between the lines of what Central Bankers mean is that they want national accounts (i.e. wallets) and using crypto/blockchain/whatever as justification. The reason being that Central Banks have unleashed interventions that 20 years ago were unfathomable. And yet, deflation is everywhere and money velocity barely has a pulse. The transmission mechanisms for monetary policy are broken...well, CB'ers broke them. So now they are looking elsewhere. Direct payments are the final frontier, and it's a lot easier to do by giving everyone an account with the CB (i.e. "wallet") and just print money there.
TL;DR - money is already digital, what central banks really want is centralized accounts that they can helicopter money into