Some parts wear out, and most simply do not

Failures cluster in the few components that move, get hot, or get handled every day.

It helps to know that a computer is not uniformly perishable. The parts most likely to give trouble are the ones under physical stress: things that spin, things that store charge and lose the ability to hold it, things that are opened and closed and pressed thousands of times, and connections that get plugged and unplugged daily. Those are ordinary wear items. Their failure says nothing at all about the health of everything else in the box.

By contrast, the core parts that do the actual computing rarely fail on their own. They tend to be outlived by the machine around them. This asymmetry matters because it means a great many machines described as dying are actually machines with one worn component and a perfectly sound remainder. Replacing a wear item is a different order of expense and a different order of decision to replacing a computer.

Heat is the thread running through most of it. Machines are designed to move air through themselves, and household dust, carpet, bedding and pet hair all conspire against that. A computer that runs hot for years is not damaged in one dramatic moment, it just ages faster, and the parts that suffer are the ones already inclined to wear. You cannot see this happening, which is why an unusually loud or unusually warm machine is worth taking seriously as information rather than as an annoyance.