Apple takes quite a huge amount from each payment (~10% I heard) so if you could hook up the same credit card using e.g. Google Pay, the bank would pay only e.g. 5% (I made that number up) per transaction.

Consumers don't see it, so this competition on iOS would benefit mostly banks if you think only about costs, but there is another side to this.

If my bank doesn't support Apple Pay, but supports Google Pay, then I can't use my iPhone to do payments because Apple prohibits Google Pay to access NFC.