I haven’t used Apple Card, but it was apparent when I financed with their previous partner Barclays that they had their own say at least in the application and approval process. It was wildly different from applying for credit in any other scenario I’ve experienced. They’re clearly backed by other credit vendors but that doesn’t mean they’re not operating their own financial institution.
Technically, a bank has a lot of rules to follow, and I do not think they want to get into that business (yet).