Why they're on the list:
They're every long-haired 16-year-old's favorite band, they've sold 200 to 300 records in their history, and almost as many joints have been rolled on their album covers.
Why they may not be in the games:
Led Zeppelin hates all technology that wasn't forged in the fires of Mount Doom. Jimmy Page might've been the original guitar hero, but he routinely shot down any possibility of the most classic of classic rock bands ever popping up in a video game.
Page is far from the only ossified rock elder to bash music games, but he's one of the few to stick by his principles and refuse to license his music to a game. Of course, Zeppelin's history is full of principled stands eventually undone by ungodly amounts of cash, which you'd think they'd have enough.
They stridently avoided iTunes until 2007, when the death of the CD was all but assured. They used to not license their music for film and television, but by 1998 "Communication Breakdown" was blasted in full in the long-forgotten film fiasco Small Soldiers. One day, Robert Plant will need a new lining for his pool and six months later we'll get the Rock Band DLC we deserve.