I haven't read the novel, but I think, it's out of respect or privacy.
I mean, even Garam would know calling out Yikyung everytime he thought about Dawon would be embarrassing and annoying afterwards. They only joke about him when they're, I guess, in the mood. None of them are 'bad' people in that sense... None of them are persistently annoying, and it isn't like they look down on Yikyung in that manner so they understand that they should respect his privacy to some extent. Like, someone wears a really bad dress that's super ugly and everyone knows it's ugly, but no one comments on it, not because they're mean but out of respect and understanding for the other's feelings.
And I also think, they can't hear things clearly. People don't think in full sentences or coherent bits, some thoughts are louder than others. They can hear him, yes, but not in the sense of having telepathy, but moreso loud thoughts that are outstanding. There might also be a way for them to drown out his thoughts.
Though, I do agree they are a little ambiguous on some things that they should be more clear on... Maybe it's a spectrum of thoughts and emotions, like how Yikyung can (later on) see Dawon's mood.
Anyway, generally, the story is pretty vague sometimes so I mean... ┗( T﹏T )┛
Ok so... I loved this. Seriously it didn't even bother me that it s a slow burn romance. Both the action and the love parts were so interesting!!!. I have just a little question for who read the novel, cause i am a bit confused. In early chapters, the servants said that they can read their master mind, which is why they knew uke was thinking blue boy was cute and garam was handsome. But the more u go on, it seems to me that it s more about reading the interior feelings of the master . Cause every time he thinks about, u know strange things or love or garam, noone comments. But sometimes they do. And in the latest ch, they talk about heartbeat. So idk of i was the only one who found this confusing. Maybe in the novel is explained better? Someone can answer this?