How come skin cancer did not exist before the existence of SPF?
Maybe, because you are making up stuff. Skin cancer has always been around.
It took a spike when modernization allowed us marketkly increase our recreation time. At the same time there was a shift in perceptions. Previoius to this change only workers and lowly workers would have tans. After a shift in the workplace in which everyone moved indoors a tan was seen as a status symbol and as such it became desireable.