Stress is very much not the same thing as work. My experience is that the most stressful times for me are the times when I'm suffering uncertainty over what I should do, or have a seemingly irreconcilable conflict of priorities, or when someone is in a bad situation and I can't do anything. The least stressful times are when I'm in the zone working hard ("work" being anything that needs focus, so might include play!).

The stress can be particularly bad for those that had no choice but to retire (due to health, job loss, or other reasons). Moving from working for money, and potentially being able to increase your income if needed, to a fixed income is tough for most people. Particularly if it was a sudden retirement and they realize the full implications of that after they retire.

I often wonder whether the causation is backwards for these stats. After all many people retire because of ill health.

Perhaps people retire because they are starting to get too sick to work.