The simple answer is they are eating more calories every day than their metabolism can burn. Cats spend most of their life sleeping, so exercise is not the answer to maintaining a proper weight. Instead the answer lies in what they were meant to eat vs what we usually feed them. Nature intended our cats to consume small pray animals not plant material in the form of grains, Veggies, rice or fruits. Their metabolism was designed to be fueled by meat and fat, not carbohydrates.
Meat digests at the same rate that your cats metabolism runs at. As the meat is converted to energy, in the form of blood glucose, your cat uses that energy to fuel all their needs. If your cat produces more blood glucose than needed, then they will be forced to produce excess insulin in order to store the excess blood glucose as fat. If the excess glucose continues, then your cat will continue to gain weight and the stress of producing the excess glucose may eventually lead to health issues.
The culprit in this cycle of weight gain is the easy to digest carbs. Carbs digest many times faster than protein and they flood the blood with glucose in excess of your cat’s energy needs. When your cat becomes hungry again they do not use the stored fat, instead they go and eat another meal. The process repeats over and over and eventually you have a fat cat that may be more prone to diabetes, kidney disease, stones, crystals and even dental disease. Since the digestion of meat is slow and synchronized to your cat’s metabolism, there is seldom excess energy flooding their blood stream and nothing therefore to store as fat. With few exceptions a properly balanced carnivore diet will maintain a healthy weight in all cats.
If your cat is overweight then you are feeding the wrong food. if the food was balanced your cat would not be overweight. The question becomes, how long do overweight cats live? The answer is not as long as if they were the proper weight. If they develop diabetes or kidney disease then life expectancy can be shortened by years. Not to mention the extra expense of treating a sick cat. Cats were never meant to be overweight. If they over consumed in the wild they would exhaust the local food supply and eventually starve.
Feed the right food, and everything will be better for both you and your kitty.