AWS has had multiple outages which were caused by a single AZ failing.

Yup, I was referring to, I guess, one of these,

- https://news.ycombinator.com/item?id=29473630: (2021-12-07) AWS us-east-1 outage

- https://news.ycombinator.com/item?id=29648286: (2021-12-22) Tell HN: AWS appears to be down again

Maybe things are better now, but it became apparent that people might be misusing cloud providers or betting that things work flawlessly even if they completely ignore AZs.