I would say that the simplicity has to do with the linear nature of most failure modes of disk drives. The overall failure curve is nonlinear due to compoundin

I would say that the simplicity has to do with the linear nature of most failure modes of disk drives. The overall failure curve is nonlinear due to compounding per time probabilities. But those probabilities are only nonlinear/cross-interacting at the point where the drive is already considered to be "a loss", and our engineering/maintenance strategies take advantage of this fact. We do not generally consider a human to be "a loss", if say, they start getting chronic back pain, which can start to precipitate other problems in a compounding fashion.