Because it is inevitable that an exec will be in a position where it is expected that (s)he knows something but does not.

There are many execs who will then react with a "willingness to stand up for themselves", and god knows that culture exists at some companies. Screening for people who have a different emotional response results in (imho) a higher-functioning team.

I think the mere fact that someone would feel the need to "stand up for themselves" in the face of a simple question says a lot about them.

The reactions seem to be to the entire interview process, and not a single question. Thus, we're talking about the "whole package", wherein the interviewer's attitude and demeanor are probably more important than any individual question.