Banks make bank-account-dollars (a form of IOU) but they can't make cash-dollars. When you withdraw, the bank has to give you cash-dollars from its pool and can't just print some. Only reason they're considered equivalent is... uh... because I said so? And they're legally allowed to denominate their bankdollars in dollars.

Anyone can print IOUs, but not everyone can legally call them dollars. That's the only advantage banks have over the rest of us.

Same on the blockchain but without the privilege of conflation. You can have a smart contract that has $100 of ether but trades in 200 shares valued at $10 each. But the blockchain systems prevent you from pretending that bank-ethers and cash-ethers are the same thing. You can label them the same but they're not the same and the system knows that. Even Wrapped ETH, a contract that literally just prints and destroys WETH 1-to-1 with the ETH it holds, i.e. a full-reserve zero-fee bank, isn't interchangeable with actual ETH.