Humans are not reliable either. Humans are much more likely to be out sick unexpectedly.
If you keep up the maintenance plan for machines they rarely break before their predicted retirement date when you replace them. And since the maintenance and retirement dates are predicted in advance you can plan for them and thus ensure they happen when you want them to.
If you keep up the maintenance plan for machines they rarely break before their predicted retirement date when you replace them. And since the maintenance and retirement dates are predicted in advance you can plan for them and thus ensure they happen when you want them to.