Why is corn the primary ingredient of expensive cat food, whereas beef is the primary ingredient of cat food from the supermarket?

my cat has a bladder problem, and the vet sold me some cans of an expensive cat food which is supposed to help with feline health issues. I forgot the brand, maybe Hills or another one of the “premium” cat foods. The primary ingredient is corn (although the cat food looks similar to regular supermarket cat food). The primary ingredient of Friskies (which is what the cat usually gets) is meat or meat-by-products. Why is it that the “healthy” cat food is made of primarily corn, which is not natural to a cat’s diet, whereas the supermarket cat food (Friskies, 9 Lives, whatever) lists meat as the primary ingredient? It kind of makes no sense.