Because they are different items which have differing validity periods. It's necessary to be able to distinguish between different items in the system.
I fly weekly to the UK using preloaded passport card data in the AL app, and check in every time on the app, and have never had an issue. The passport number is the passport card number.
I fly weekly to the UK using preloaded passport card data in the AL app, and check in every time on the app, and have never had an issue. The passport number is the passport card number.
That just shows how indifferent management are to customers. They're aware of this for months and did nothing to fix it. They're happy to inconvenience customers and make them collect the boarding pass at checkin.