It's the concept of the show. They encourage the guests and also the casts to use informal speech (반말) so that they get to interact with each other as schoolmates often do, leading them to be comfortable enough to tease one another. It's like the setting is they are in the same school and they all are friends.
Watching this now…and I am rooting for ZOFGK for Worlds 2024 again…this is the 3rd year they have been together, the longest staying roster to date. Whatever they decide, I will support them, though I wish they would stay together- but I am already grateful for the 2 internationals they have given us…all the deep runs they have achieved in the regional and international events…and just staying together- hats off and with utmost gratefulness to this roster,🫶🏻🫶🏻🫶🏻💐💐💐👏🏻👏🏻👏🏻