Credit card companies are mostly parasitic middle men but I have a credit card that I use for most payments and pay off the balance every month and effectively get 2% cashback with no interest costs.

The actual rates charged to merchants are variable, and typically higher for rewards/cashback cards. Basically, other people are subsidizing cards with higher benefits/cash back (of course as people shift, the merchant raises prices to compensate for the higher average fee, or just charge an additional Z% higher than your cash back)