Does anyone know why KRAFTON does not publish their games on GOG?

Their most popular games seem to have a strong multiplayer focus, which GOG isn't well-suited for. And they might not care about the little bit of money (compared to their $1.9 billion revenue of 2024) they'd make from GOG.

The Callisto Protocol would be a good fit, but I've emailed Striking Distance Studios both in 2022 and a few months back to no avail (never got a response).

Seeing that they're a Korean publisher it's probably that they're not too fond of DRM-free and having ownership over (the copies of the) games you purchase. Like most other asian publishers.

Any kind of always online - even for singleplayer-only games - also doesn't go well with the concept of offline installers, obviously.