In the early days of central banking, one primary objective of the new system was to take ownership of the public’s gold, so that in a crisis the public would be unable to withdraw it.
Gold was to be replaced by fiat cash which could be issued by the central bank at will. This removed from the public the power to bring a bank down by withdrawing their property. A primary, if unspoken, objective of modern central banking is to do the same with fiat cash itself.
There are of course other reasons for this course of action. Governments insist that they need to be able to trace all private sector transactions to ensure that criminals do not pursue illegal activities outside the banking system, and that tax is not evaded. For the government, knowledge of everything individuals do is necessary control. However, in the monetary sense, anti-money laundering and tax evasion are not the principal concern. Central banks are fully aware that the financial system is fragile and could face a new crisis at any time. That’s why cash in their view must be phased out.