A generator that performed fine during last year's outage can still be in worse shape today. Batteries degrade gradually, not all at once. A transfer switch can wear a little more with each use. An exercise cycle completing successfully month after month doesn't guarantee the unit would carry a full real-world load the next time it's actually tested by an outage. Storm readiness isn't a one-time achievement — it's a condition that has to be reconfirmed before each season that's likely to test it.