This question came up on Reddit and the answer is post-production skin whitening and heavy makeup. If you see the same actors in films or older kdramas you see that in reality they are much tanner and have (gasp) visible pores.

I found an article with amazing photos of Korea in the early 1900s but the publication is garbage so I won't link here. Instead here's the photos and it's fascinating to see the contrast with the idealized images in sagueks, esp. skin tones. For more info, check out the alt text. #kdrama