Since we're on the topic of authentication, how about the fact that they are not recoverable? You cannot reset a password on the blockchain, nor can you call the blockchain and prove you are the rightful owner of any inaccessible/stolen funds, nor can you take the blockchain to court to return your funds. You are SOL.

Just about any service that banks do are great examples of other things that math itself cannot do for you. These are all reasons that people still overwhelmingly use banks.

Banks do work to integrate with other societal systems in meatspace, build infrastructure, manage exceptions, comply with legal expectations, provide service, build and maintain partnerships, etc. Cryptographic ledgers don't do any of this, they are inanimate.

Hm. There are some things that banks do then that math can't do. But advanced cryptocurrencies have smart contracts that allow DeFi systems to handle many typical banking functions.

The primary things that people use them for, to store money or sometimes distribute it, or act as a system of record, have been made obsolete though.