Because in the world of business there usually aren't any easy or singular answers.
Asking open ended questions allows an interviewer to test your ability to react to difficult problems and changing circumstances - it also makes it harder to prepare in advance.
Interviewer: Why does the boss have the back corner office?
Candidate: Because he deserves a good view.
I: But our boss in scared of heights always keeps the blinds closed. Yet he still insists on that office.
C: Does he ever open the blinds?
I: He does during some meetings, but sits with he back to the window.
C: So, he wants the view its not just for him. So he likes to demonstrate his influence to vistors.
And so on...
Alternatively, the boss may not ever be in the office, it might be the only office with a private bathroom, etc...
The point is that instead of testing your ability to calculate an answer, its about having you explore alternative options.