Increasing your connection with someone else through an inside joke cuts off anyone who both hears it and isn't in on the joke. That's why the last time georgeorwell encountered inside jokes was in high school: someone else made the joke, he didn't get it, but everyone else did.

Of course georgeorwell has almost assuredly heard inside jokes since. The difference was that he got it and it didn't occur to him that other people wouldn't.