Main housings break more often from normal wear and tear, but they also break if dropped or exposed to the elements. As most people have already learned, plastic becomes brittle over time and with exposure to direct sunlight, which is why car keys can be broken or cracked.
Each time the vehicle is ignited, turning the key in the ignition also puts pressure on the section where the key blade meets the plastic housing. Even in specially formulated plastics, cracks will appear, and the repeated action of turning the key in the lock or ignition will cause the key compartment to rupture.
Key shells that seem to break in a queue may seem like a scheme to give the manufacturer more money. After decades of using steel keys that last relatively indefinitely, the use of plastic for key handles seems a step backward in terms of durability. But the cases of broken or cracked keys are the least worry of a car owner when it comes to high-tech security in the key and the immobilizer system installed. Because of these characteristics, we may have to live with compensations.
If you go to a dealership to replace a broken key fob, its price will make you think it was a significant machine component that needed CNC machining and a gloss finish. It is usual for dealers to charge hundreds of dollars for a replacement key.