That's why it's called "electronic" or "digital" cash

Physical cash is nice locally, but you can't use it to transact instantly over large distances with anyone on the planet

A few other downsides with physical cash:

-Can't carry arbitrarily large amounts.

-Vulnerable to confiscation and theft.

-More difficult to verify authenticity on-the-fly.

-Issuance is centralized, unpredictable, and doesn't require PoW.

-Not as easily divisible (Need to carry specific denominations and change)

-Can't trustlessly exchange for crypto (no atomic swaps)

-Can't have multiple people with simultaneous ownership and control over the same cash (multisig)