Disney's IT systems are ridiculous and basically designed in ways that make communication between different systems seemingly herculean.
DVC, Disney Ticketing, Disney Dining, etc are different systems that you'd think would easily communicate with each other but they don't. It'd be a Big Deal for them to work on a system that works at a point of service (POS) and weaves its way through multiple systems to kick back a verification or denial. Could they do it? Absolutely! Will they do it? Doesn't seem likely because it'd cost money and it's cheaper to just push a digital card that works for most guests.
But if we're going to dream realistic dreams then a system that automatically applies a discount would save everyone time. Instead of a waiter verifying and adjusting, the system could just auto apply a discount if the primary reservation holder is entitled to a discount. Same thing at merch checkouts, like Rose Golf said, people should be able to tap in but also have discount auto applied. This *does* come together in the app where mobile checkouts for merch and TS auto apply discounts, so we know they can do it.