Basically it's because people misunderstand what Mensa is about. It's just a social club for people who score in the top 2% of the population in IQ tests - nothing more! But somehow people expect them to use that (usually) one meeting per month where they want to socialize, to go out and save the world and solve all its problems!

Of course it does not help the situation that you find, like in any group of people, those who are arrogant about their "strong points" (and being smart, they manage to do that very well!) I've only attended one annual gathering and a few monthly meetings; so I can't speak for all the Mensa chapters, but I never heard them discuss their IQ's or such things. They might have interesting intellectual discussions or get people giving interesting talks, but in general the just party really hard and have lots of fun. And then they go home and spend the other 29/30 days of the month going on with their lives where Mensa doesn't even feature (and some might even go out and do things to save the world!)

Another major reason for criticism is of course that people see the IQ requirement as a criticism of their IQ if they don't qualify. But it's a just a social club - you don't lose out on anything if you're not a member! You won't hear me whining about not being accepted into some millionaire's club because I'm not rich!