How is the sun so bad for us?

I managed to get a mild sunburn yesterday and it got me to thinking about the risks of sun exposure. I’m fair skinned and pretty diligent about sunscreen, but I often manage to burn every now and then even when I’m careful.

My question isn’t about whether or not sunburn is bad for you, but how it is that we are so poorly adapted to a pretty constant and vital part of our environment. People have been out in the sun all day presumably for all of human history, but in recent years we have learned that a few minutes of unprotected sun exposure a day is the only “safe” option.

Animals, including pigs, seem to adore sunbathing. Pigs have skin that is similar to ours, do pigs get skin cancer? Are they better adapted to sun exposure than we are?

How have we not evolved to be safely exposed to more than a few minutes of sunlight a day?