Let's say you are a bank and you make $10 on each $100K transfer. If customer disputes a transaction and you must return the money, you lose the whole amount and twice as much on lawyers, internal audit, compliance people working on the case.
With this math you can't afford the risk if it is more than 1 in 30000.
For many European banks the math is even more brutal.
No bank got fined for not root checking, correct. However banks are on the hook for unauthorized transactions. And "unauthorized" means different thing in different countries.
In some jurisdictions if bank can prove that transaction was made with customer's key then customer can not demand their money back. That's the best case, but there are only few of such jurisdictions and even there the burden of proof is on the bank and it costs a lot.
In other jurisdictions bank must reverse a transaction even if it was proven that the transaction was signed with a legitimate key, but the key _may_ have been stolen.
In some jurisdictions (i.e U.S.) banks are required to reverse a transaction at a customer’s request, even if the customer does not dispute having made the transaction.
In any case dealing with all this is too expensive and risky.