Afterwards I realized that its entirely a subconscious alpha dominance thing. There's this unsaid very primal, tribal power trip that goes along with the interviewee vs employer relationship.
As the interviewer you want to be holding the power card - you sit in a position of power and have other people dance around and do what you say in order for you to be able to judge them and make them prove their worth to you in order to join your tribe.
Its really one of the few times we get to break down our democratic social structure and revert back to this primitive social order in adulthood, so its a pretty important ritual for alpha-types.
When someone comes in and disrupts that natural boundary it becomes offensive as they've unconsciously told you "I don't respect you as a leader" in front of your staff and team. The interviewer then needs to re-affirm their ego and dominance over the tribal unit in a public display.
If you look at all these articles floating around about judging candidates and "top grading" and you look at it from this tribal alpha-dominance perspective it really breaks down some of what we assume is necessary in hiring. I think the whole system is based around some silly ancient ritual that we haven't been able to shake from our culture.