> It is found mostly in people who avoid/block the sun and not eg people who work outside everyday.

This is classic confusion of correlation and causation.

People with lighter skin are aware of the increased risks of sun exposure and avoid/block the sun. Further, a lot of outdoor labor in the US is now done by people with darker skin (e.g. immigrants from Mexico with indigenous heritage).

Finally, sunblock isn't perfect. At-risk people can reduce, but not eliminate, the chances of developing a tumor. European-descended people are living in climates far south of where they evolved light skin, so cancer is inevitable.

> Also, tobacco smoke seems to be an exceptionally weak carcinogen

So is radon. What's your point? It may be weak, but it causes a massive number of avoidable tumors and deaths per year.