Bank servers have bugs in them!

To make a digital payment a large infrastructure needs to work flawlessly. From working card reader device, to a working internet connection, working electricity, and working, not overloaded back servers. So many things have to go right, that credit cards often fail.

Cash is just paper and coins. You don't need any infrastructure to use them.

Software Bugs

The more lines of code a program has, the more bugs it has. This is just plain axiom of the software world. It doesn't matter if it's built by an amateur or a professional in a corporation, that has a contract with the government. Bugs are inevitable.

Often those bugs manifest themselves in failure of operation in critical infrastructure. A bank server can suddenly get confused and crash. Needing time to reboot to a working condition. A card reader device can suddenly forget how to talk to the bank and so on and so forth.

None of those issues exist with cash.

Physical Infrastructure

High-tech firms often like to mislead people into thinking that internet is a magical cloud somewhere in the sky. In fact there is nothing magical about it. Internet is a net of connections between various computers. Some are your phone, some are the servers that have software on them for you to interact with. And the connections are done in vast majority using normal cables.

Even if the software on those systems will run well, any environmental damage to the hardware ( cables and servers ) will render the entire thing incapable of servicing your transactions. And don't forget that some people just like to vandalize infrastructure.

With cash, none of this is a problem.