You'll never hear the real reason. So if you have an issue with this fact, don't invite the candidate to an interview. If you don't have an issue with it, then why ask? They'll never say "because I'm constantly looking for better opportunities and will jump ship when I can".

This is what most of those questions on the list have in common: both sides of the table know that the answer will be contrived BS. They are a waste of everyone's time.