It's actually a bit more complicated than simply using sunscreen
There was a recent report that sunscreen, while good at reducing sunburns, also blocks the body's absorbion of vitamin D from sunlight. Among other things, vitamin D appears to reduce some cancers, including skin cancer. In effect, sunscreen reduces the chances of sunburn but not necessarily all types of skin cancer. There are a lot of people who work outdoors all of the time. This isn't a trivial issue for them. Malignant melanoma kills about 7000 Americans a year. Not exactly a laughing matter.