I’ve had the same experience, with paper shredders - particularly the Fellowes brand, in the UK - which all have a nylon (possibly some other plastic) cog, somewhere to overheat, then break.

In this case, I think it's so they break before the bones in your hand do.

They’re built out of plastic because it’s $0.02 cheaper than a metal part and 99.9% of them last through the warranty period.

It’s not deliberate that they make stuff that fails, they build stuff that lasts only as long as it needs.

There's a single nylon gear. The rest are made of metal. They've been using a sacrificial worm gear since the 1960s.

KitchenAid switched to all-metal gears on their high-end models fairly recently. In the past, all models had a nylon gear, but the new high-end mixers use electronics to protect the motor.