I guess that's because the atmosphere of Sieon's house has remained the same since he lived alone until he lived with Namwoo, nothing has changed. For example, Sieon is a neat and organized person, if there were Namwoo's things placed on the shelf or wherever then Sieon would feel like there was someone living with him, he was no longer alone. Indeed, the clothes and toothbrush were placed in the same place as his but still, didn't change the fact that Namwoo didn't have his own "place".