Because that was a game, this is real life (well in the bl) and he reincarnated as a side character with minimal info in the actual game not as one of the main characters ofc he wouldnt know a random patient or all the patients the character he reincarnated as treated
he had to be taught the concept of dungeons & about the enemies in low-rank dungeons. how did he not come across that while playing the game
like idk, maybe the genre of the game was a visual novel or something and not a game with actual gameplay
Keep in mind that he also transmigrated after the end of the game and beating monsters in a game is so much easier and safer than in person experience.
why is he so clueless? didnt he play and beat the mf game??