They have Apple Card so presumably they are capable of providing financial services like loans on their own, why bring in a third party who will take a cut and force your consumers onto their app with a bad reputation?
Apple Card is the result of a co-branding deal between Apple and Goldman Sachs (which is currently in the process of exiting the deal and being replaced in that capacity by JPMorgan Chase). Apple provides the branding and some minor fintech features; Goldman (or in the future Chase) issues the card and is on the hook if people don't pay their credit card bill.
Presumably for this particular program using Klarna was cheaper because Klarna already specializes in precisely this kind of transaction.
I think Apple gets sweetheart deals so their partners often lose money. Klarna likely sees this as a marketing coup to have their predatory sussness promoted by the Apple halo brand.
If there was profit to be had, I'm sure Apple could buy a small bank and spin it off as a subsidiary with little drama, it's just not the best of their options.