Because they don't have to. Their stomachs can cope with almost everything. In contrast to humans, dog saliva does not contain any digestive enzymes. The saliva serves exclusively to ease the path of the food through the oesophagus. It follows that dogs only reduce their food to a size which allows them to swallow in comfort.
Relatively large chunks of food land in the stomach as a result. Initially, this poses no problem, as a dog's stomach is extremely elastic - its size always depends on the amount of food absorbed. The food remains in the stomach for between six and twelve hours before proceeding to the intestines. The food particles may not exceed a diameter of 1.5 mm in order to be able to pass through the pyloric orifice, or the end of the stomach tract. If the particles are too large, this increases the risk of stomach distension or twisting.
Large croquettes force dogs to reduce the size of their food by using their teeth. In this way, you can ensure that smaller particles reach the stomach - and can be digested quickly and efficiently.