There are more separate elements of lower quality testing than CPUs (which go through serious qa and binning). For example capacitors on motherboards break/explode, PCB traces degrade/crack more often than a CPU will break.

CPUs have a lower failure rate than motherboards, which makes sense if you think about it. (One is an IC, one is a complex assembly of many ICs and passive components.)

Anecdotally, I'm still running a computer I mostly bought in 2011, i5-2500K still going strong, but the motherboard has a few issues: one of the PCI-E slots works sometimes, one of the RAM slots is completely broken, and the RTC seems to lose a few minutes a week.

CPUs should go pretty much forever, provided they're run on clean power. They're a little square, entirely encased. Motherboards are bigger and with physical connectors you need to use force on.

Back in the day the story was that a knockoff manufacturer of capacitors used an incomplete recipe—the caps started out fine but didn’t last. Wikipedia says the story is unverified but still includes it under the “Investigation” section:

https://en.wikipedia.org/wiki/Capacitor_plague

No moving parts, small and stable voltages, relatively stable temperatures, no capacitive sections to experience electromechanical strain. There's essentially nothing that can "wear out" in a CPU. Basically the only things that can degrade a CPU are if you run it on a noisy voltage source (unlikely, behind a power supply), or if you regularly turn it on and heat it up a lot and then allow it to cool. But if you keep a CPU well cooled or just run it constantly it will last essentially forever.