Can’t they just track everything you buy using your payment card?
That's why.
They can use them for all sorts of tracking and pair the data up with any data that they buy from other places. Also, many 'loyalty' cards - e.g. Tesco's one, only works really against the consumer as the customer is penalised for not having it. The prices only drop to normal prices when the card is used, with the higher price for those without a card.
Hands up, I haven't watched this video, but I guess it will explain better than anything I can - https://www.youtube.com/watch?v=psta0qkwk7M