I think the problem here is that Apple wants payments on the iPhone to be secure, easy and private. If they gave banks full access to the api, they would do what’s in their best interest (reusing CCs numbers accross vendors, making it harder to use more beneficial cards, less transparency). This would mean that paying on iPhone would be considered harder or more inconvenient then using a card because many consumers don’t know the difference between Apple Pay and their bank app’s payment system. It would also mean that fewer banks adopt Apple Pay, which is already not that great in many countries.

I’m honest not sure if opening up the system would be a net benefit. There are certainly advantages to keeping it closed but we also miss many great opportunities with NFC because of that. I would expect Apple to open it up eventually, when users become used to using Apple Pay and expecting the same convinces from their bank app