Three main reasons:
1. Software that becomes more demanding / not as well optimized for low-spec machines.
2. Bloatware that clogs your drives, system services and autostart. A competent user can actually handle these effects pretty well, and restore even older machines back into surprisingly good condition.
3. Decay of the hardware, e.g. drives going bad. Running at lower temperatures can increase the longevity of most hardware significantly.
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