I always wondered about this too, but simply accepted that it's a TV show, and for the sake of the show (and likely the fans) there was just little chance any of the main characters would be adopted, at least not long term. I could see an episode centered around the adoption of one of the leading friends (probably ending with the friend back at the house for some reason or other). And frankly, am kind of surprised there hasn't been an episode like that, with the exception I suppose of "Emancipation Complication". But that wasn't what I had in mind. Anyway, all this clarification is quite enlightening, and makes sense. Maybe it's not just because it's a TV show.
As for the background friends, most of them we see only every few episodes. It's been my theory that some of them
have been adopted, and that's why we don't see them for many episodes at a time. When they return to the "background", they're just back at the house. Heh, just my theory though.
