A traveller tries to leave a few thousand won on a restaurant table in Seoul. The owner catches them at the door, cash in hand, assuming they simply left it behind by mistake.
In South Korea, good service is viewed as something you have already paid for in the listed price, not something requiring a separate reward. Offering a tip can occasionally feel, to the person receiving it, like an implication that they would not otherwise have done their job properly — the opposite of the compliment a visitor intends.
A small number of five-star international hotel chains in Seoul, catering heavily to Western business travellers, have begun accepting tips for services like porter assistance, in line with the expectations those guests bring with them. Even there, it remains optional and far from universal, and a local-style establishment just down the street will likely have no such custom at all.