Why does my cat drink her food?

My cats teeth are healthy, and she sometimes eats the dry, crunchy food, which helps her teeth stay clean. But I regularly feed her the moist foods like Iams and Fancy Feast. With most of these meals, she consumes the gravy or sauce, and usually leaves the solid chunks of meat, fish, chicken, turkey, or whatever. Why would she do this? And if I try to feed her less expensive or generic foods, she wont eat them at all. ("Furrbie" is about 12 years old and otherwise of great temperament, and good health. She is allowed to go outside, in good weather.) She is otherwise a no-problem pet. This has been expensive, and irritating, because I end up dishing out 4 or 5 little cans of cat food each day! Help Please!