Cats prefer to eat the same thing every day. I think they do it because it is safe. Their thinking goes something like this: the food I ate yesterday did not hurt so I will eat it again today. Cats are very much creatures of habit and love everything to be as predictable as the day before. Just try changing their routine on anything and you will soon discover an unhappy cat.
Cats do appear to enjoy treats. Treats are usually heavily flavored and overpower their sense of taste. However, many of the flavors that are used on treats are not appropriate for use on daily foods. Many flavors are high in sodium or are unbalanced in other nutritional profiles and are best used sparingly.