I see the following problems with the idea of hotels issuing one time only keycards:

Surely food and other items (clean towels and sheets anyone? coffee and minibar supplies? soap and shampoo from the hotel?) will be delivered to rooms, presumably in contactless fashion with the items being left on the floor (or for small items in a plastic bag tied to the doorknob like Hyatt does). Yet when one tries to open the door and bring the stuff into the room, there's a good chance that the door will close behind one, leaving the person in the hall. [This has happened to me often and room service trays can be the worst things to bring into the room without ending up in the hall on the other side of a locked door. I've trained myself to put keycard in pocket (or in hand, but this makes it harder to juggle that tray with stuff that can spill) before I ever open the door. ] At this point, presumably the person has no option but to walk to the front desk (or find a house phone) to get a replacement key card, resulting in needless contact with hotel staff and other guests.

OTOH if the answer is that other family members on the other side of the door, in the room, open the door for the person trapped outside in the hall, then by the same argument most family members can go out and about at will (depending on how the resort is arranged) providing that there is always one person left behind to open the door when family members return. There's no effective deterrent for quarantine violations if the room contains more than one adult.