I'm not talking about contracts.

Every bank is regulated by their central bank issuers. These banks have a mandate, in fact in most places it's law that depositors funds are not touched i.e. used for anything other than just holding in the account.

Where the interest earned on savings comes in is rather inflationary hedges, so very very small percentage in comparison to a fixed deposit or, as you say, contract for say 2 years at 10%. That money fixed for two years by the customer can be used by the bank as investment vehicles.

So savings accounts aren't loans to the bank. It's YOUR money. In fact most offer savings accounts as an extra service, because it's not a generator of income for them unlike their other banking services.

Many people will fix their money into a higher yielding CD like 1 or 2 years. This is money the bank has access to.

Now I'm not saying bankers don't skirt the law and steal customer funds (this happens) but that's why there is the law in place to protect customer deposits vs customer investments - after all, the central reserve can literally print back the money you lost from the bankers.