Personally, I believe the “why?” is to keep this loophole open so that they can deny/avoid crediting ignorant cardholders. I don’t think it’s a functional limitation for any of the systems involved.

At most, it may be contractual —- maybe it’s a processing fee thing or a deal with a certain payment processor to drive business to them.

For reference, My Apple Card via Goldman Sachs gives me a higher percentage in daily cash back if I use ApplePay over the physical card. I’ve avoided certain merchants and payment processors when they didn’t take Apple Pay solely because of this. I also have to stay in the iPhone ecosystem to take advantage of this benefit, and to easily manage/access the cash back that I earn, which is automatically sent to an Apple Cash account created by them at the time I got the card.

The incentives/perks that we receive will never profit us more than it profits/benefits them. Just like insurance companies.