Reply to Message

So that the device has a longer useable life, which translates to greater value to the customer.
Unlike a calculator, TV, radio, microwave or MP3 Player, a computer is a device of significant cost, with modular parts that degrade at differing speeds. The parts that degrade fastest are considered "consumable" and can (usually) be (easily) replaced.
By designing the device itself to be consumable, the value of the device is decreased.
This structure is of financial benefit to Apple, not the customer.
. . . . It's also environmentally unsound, come to think of it.