Most people worked outside in the fields for half the year and/or died in their 20s and 30s.

Maybe Europeans, on average, walk around more? In my experience, much of the North America is far too sprawled out to make walking viable for a vast majority of people.

people are indoors during peak sunlight hours far more than ever in the last 100 years

Common misconception. In fact, the population barycenter of the US is further south than Athens, Greece, and lies about halfway between the Strait of Gibraltar and Lisbon, Portugal.

Even more then.

Possibly better food supply?

They're also lighter skinned on average than the US. Your skin color makes a difference in absorption. Native European's skin evolved lighter for a reason, and that was to absorb more of the sun's benefits in a cloudier climate. The opposite is true for dark-skinned individuals; they do better in sunnier hot climates because their skin protects them from over-exposure.

No?

Madrid is about the same latitude as NYC. Paris is significally to the north of Montreal. London is about the same latitude as Calgary.

Even more then. They should suffer from Vitamin D deficiency way more then, but we've been living there for tens of thousands of years.