My office was in an academic building, right on a hallway. Keeping the door open would have meant letting all the bustle and noise of students changing classes in to interrupt me frequently, not to mention significantly increasing the number of confused students who would try to walk in thinking my office was a classroom.

Frankly, I consider the expectation of constant availability to be another abusive part of work culture, very closely related to the "everyone must return to the office so I can see for myself that they are in their seats (which means they must be working)" mindset. Let people keep their doors closed to promote focus.