I simply haven't joined because I haven't gotten around to it.
As for why, there is some ego-stroking going on, but as a friend
of mine once said, intelligent people (not just in Mensa) have the
ability to mind-fuck themselves into oblivion. They often have as
much trouble socializing as the mentally ill. Being in Mensa is a way
to be around people who feel the same way about their troubles
dealing with "normal" worldviews, a way to socialize, and a way to
interact with one's peers.
In some ways, it's much like being on this list.
Noctem