Because it would probably have vastly, vastly slowed down the processing of incoming passengers. One of the first Geo posters to enter under this system described the process when it included app downloading and registration. He was estimating 1-1.5 hour wait times unless you were luck enough to beat your planeload of people to the queue.
Would you prefer to hang around home waiting for an SMS, or be physically stuck in a queue for an hour and a half after an intercontinental flight, quite possibly with tired kids in tow. Or listening to other peoples' tired kids.