Because including "Mensa Member" on your cv or resume constitutes bragging about your IQ. And bragging your IQ is a bit much like bragging about your height, shoe size or *ehem* "endowments."
It is no worse than putting "Member, ABA Student Section" on your resume. It doesn't mean a whole lot, but it is an organization that you are a member of. It doesn't constitute bragging at all. I think you are reading way too much into it. Some people (on this board!) put their hobbies on their resume.
I would propose that a great many people who top their classes aren't necessarily the most intelligent of their classmates.I agree.
By the way, have you ever actually been to a Mensa meeting? It's slacker central - not a glowing attribute to include on a resume.
Better than gunner-central...