You are comparing apples to oranges. Many banks offer custody services which are off balance sheet for them. That is they don’t count as assets or liabilities because there is no use of the custodial securities by the bank.

The big difference is that you pay for the privilege of using a banks custody services. That’s why most people don’t use them for cash, it’s equivalent to putting cash in a safe deposit box, it depreciates even in deflationary times.

Signatures crypto clients were not doing that. They were loaning money to the bank in order to achieve yield. No matter who it is you loan money to, you have counterparty risk, banks just happen to be one of the most regulated debtors in the world and depositors have unprecedented creditor rights.

No crypto firms have that same level of security.