Depends on how they're built, and building them takes experience. Plus, if their MTBF is long enough with enough hot spares, you can rotate the problematic ones out fix them while they are being replaced from the hot-spares pool.

When you are not budget constrained, and building things for businesses, a little overengineering goes a long way.

I have a Xerox 7500DN color laser printer next to me, and it's working for more than 20 years at this point. It has gone through a lot of spares, but most (if not all) issues are from parts wearing down naturally. Nothing breaks unexpectedly on that. Same for robots. Give enough design budget, overengineer a little, and that thing will be one hell of an ugly but reliable machinery.

When you work with real "industrial" stuff, the landscape is very different.

All moving parts degrade. A nice thing about machines is you can service and refurbish them to like-new condition.

There are options to deal with your shitty knees, hip, and back, but none of them get you back to 100% of your original capabilities and, carry an element of gambling, and will involve the kinds of painkillers that can ruin you far more comprehensively than a shitty joint will.

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.