UV intensity changes with the seasons. Expose your skin to the sun when UVs are lower and your skin will adjust to the higher UV levels slowly over time.
Before the industrial revolution, lots of work was outdoors, such as farming. Changes in industry drove folks to work more indoors, in factories and offices. Combine that with the culture of working all year and vacationing in the summertime. Folks stayed inside when UV intensity was manageable, and went outside for the bulk of their vacation, when UV levels were more intense. No wonder people think they need a product to protect their skin at all times from UVs.
I’m not against sunscreen. I’m against unnecessary use of a product that encourages more use of said product. At high altitude, I’ll sometimes put zinc on my nose and ears, though I usually wear long sleeves to protect the arms.