its the natural wear down of any operational machine. you can't run a machine without creating wear.

Unlike strictly mechanical machines that are repaired and maintained by an independent operator, electrochemical-mechanical (aka Biological) machines, continuously gradually repair themselves with micro-patchwork fixes instead of whole component changes (though to a limited extent that is possible now with the aid of independent operators, called surgeons and medical teams), and the rate at which those fixes occur happens varies on the available resources, how easily those resource can reach the area (good luck on joint repair), what else in the total system needs repair, and how good the total system is overall operating.

But the repair systems themselves also incur damage, and poor patchwork makes future repairs more difficult. the ability to process materials lessens, the marrow creating the biochemical slurry that transfers resources throughout the system lessens in quality as damage builds up. Harder to repair systems get filled with quick filler scar tissue that doesn't properly mimic the function of the damaged tissue, and eventually one or more of dozens of critical systems can no longer maintain to operations at a minimum necessary levels of function, creating cascading terminal stresses on the remaining functioning systems.