RFID tags are still to expensive in the supermarket context. The margins are usually razor thin (at least in Germany) and the products are quite cheap. Even when buying RFID tags in bulk they are too expensive.
Assume that having a single cashier costs $15 per hour. Assume that an RFID tag costs $0.05. Assume that in average a cashier handles 1 item for only 5 seconds (including payment).
That means that a cashier can handle 720 items per hour, and that costs the supermarket $15. Having RFID tags for 720 items would cost $36. That's still a ~$20 dollar difference, which still amounts to ~$0.028 per item.
I would be willing to pay that if that means i can forgo the checkout lines.
The big problem is: Most people are not willing to pay for that :) Depending on the product, 1-2 cents might be your whole margin.