Thanks for pointing that out.

In the winter, only 10 percent of the body is exposed, and nearly 2 hours of sun exposure at noon is needed to produce a sufficient amount of vitamin D.

Source

I thought less time was required.

Eat fish oil instead of fish if you're worried about mercury.

As for d3 only being found in only a few foods? That can't be correct.

Nope, it isn't.

But you go on drinking milk if you like, people do lots of weird things.

We have absolutely zero evolutionary need for milk past infancy. It's all culture and marketing.

Just because we do those things you mentioned doesn't mean they're necessary. In fact, I assure you, neither clothes nor cooking are strictly necessary for an individual's survival. They're convenient and comfortable, but not physiologically necessary. Obviously we'd freeze in cold climates, but there are plenty of places on earth we could survive well enough without.

We do need vitamin D to survive, but you can literally just get it directly from sunlight. The more time you spend inside, and the more thoroughly you clothe yourself, the more likely you are to need to supplement it. Simple as. You only need 30m outside 3x a week (2hrs in winter regions).

Another funny thing about milk, is that all the natural vitamin A and D is removed in processing. They literally have to re-fortify it again, so the only reason it's billed as this "healthy" thing is because it's basically a lab-made liquid vitamin.