[Thorrez]

If I operate a website, I might have some confidence that my key hasn't been stolen in the last year. But I might have much less confidence that my key hasn't been stolen in the last 20 years.

Certificate expiration means I don't need to worry about that second case.

[oarsinsync]

That’s only true if your key is regenerated each time you request an updated certificate. This is not mandatory, and there are lots of guides on the internet for generating a csr from an existing key.

[Thorrez]

Sure but I don't think that's generally done or recommended. I think people only do that if they have certificate pinning, which on the web is pretty rare and getting rarer as browsers drop support for HPKP.