It would explain why there was a surge in positive rates in Arizona when summer hit. Everyone was inside where the AC was, not getting sun.

The correlation observed for many things associated with vitamin D, is that healthy people spend more time outside than unhealthy people, not that being out in the sun is an inherently healthier activity. E.g. runners/athletes spend more time outside than those who are not, and the increased vitamin D exposure is mostly incidental. People with busier social lives tend to spend more time outside than those who do not. Etc.

For a lot of people who live in the Northern Hemisphere who are about to go into a period of long, dark nights during winter, your advice of "get outside, do things outside under the sun and let your skin do its work" is not really as helpful as taking vitamin D supplements.