Why is Trelawney always in House Harem? Mcgonagall occasionally visits Gryffindor tower in canon, but is not a permanent fixture. Is that because there are no prefects? But the members of House Harem are all married to each other (well, to Harry anyway), so why would they need supervision? Unless she's there to make sure they do their homework and brush their teeth, what need is there for her to be there?
Author's response
Two main reasons:
First, she has found that she enjoys it. Better than spending the evenings alone in the North Tower with a bottle of sherry and her Inner Eye.
Second, remember that other houses have prefects, and Harem doesn't. Without the two professors, there are no authority figures at all preesent.