Some datacenter failures aren't related to redundancy. Some examples: 1) transfer switch failure where you can't switch over to backup generators and the UPS runs out, 2) someone accidentally hits the EOD, 3) maintenance work makes a mistake such as turning off the wrong circuits, 4) cooling doesn't switch over fully to backups and while your systems have power, its too hot to run. The list can go on and on.

I'm not sure why this is a big deal though, this is why Amazon has multiple AZ's. If your in one AZ, you take your chances.