Because the cards aren't different cards.

In my case, I have a personal card and a shared card from the same bank. The card type is the same, one just happens to have my spouse as a co-owner.

Some banks do allow you to pick a card look/image. Most don't.

But whatever the case, Apple really should allow tagging cards in the Wallet with a small icon/emoji/something. It doesn't need to be fancy - just enough to visually distinguish two otherwise visually similar cards.