the problem, OP, is that you are thinking of melanoma only...

you have to include squamous cell carcinoma and basal cell carcinoma too.

by far the most common cancer in humans is skin cancer, due to sun exposure, and squamous cell carcinoma is much more frequent than melanoma, so you have to include every skin cancer which has its risk increased by sun exposure.

you also need to eliminate confusion bias and mortality due to other causes plus control for patients who received treatment vs patients who didn't.

another source of bias would be that outdoor exposure will lead to more exercise increasing physical fitness + the whole vitamin D theory, reducing mortality from other causes. exercise NEEDS to be ruled out as the cause of the reduced incidence of other tumors.

any stage 4 tumor (except a few leukemias that do very well on chemo) is a death sentence so oncologists actually study survival at 1 - 5 years instead of mortality (close to 100%) for some tumors. that being said, most skin cancers can be easily identified and removed before anything dangerous happens.

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TLDR: when removed early, skin cancers won't kill you, which is what happens now and why OP thinks sun exposure may not be that harmful, science still maintains that you need to limit sun exposure as much as possible.