Why loyalty cards suck

I do not do store credit cards and a couple of the places I go to hawk them incessantly. It is freaking annoying. Sometimes when I make a large purchase they are all like, “You could save $15 percent today if you get the store credit card,” and then when I decline I get the, “You crazy bitch,” look - how could I give up ALL THAT MONEY. Hard to explain that the companies would not be pushing these cards down my throat if it was a big loss for the company. What mysterious reason could they have to try to hook me on their credit card?