I don't understand this sentiment. There are race/ethnicity-restricted clubs, men-only clubs, athletic clubs, equestrian clubs, farmers clubs, car ownership clubs. However, when it's about IQ (something which you say you hold in disdain), suddenly out come the insults. Insecure nerds, right? They only have their insignificant test scores, they must be a despicable lot!
Did you ever stop to consider that people might be seeking something different, that this is their attempt at finding other people with similar interests? There is nothing wrong with one valuing their own high IQ test score.
Disclaimer: I have never taken an IQ test, but if I did I probably wouldn't get into Mensa, extrapolating from previous test scores and grades.