Actually, it can be. Everybody "sees" the Greek problem but it's almost universally shrugged off as too minor to derail the bull (sort of sounds like sub-prime which was also too small to worry about).
I guess I should say the bears see it as the black swan and there are so few bears anymore a "Greek" blowup would definitely qualify. Only a handful see it as any major threat so the bulls would absolutely be surprised if it blew .
up and eventually derails his overvalued market will likely be something else, either something not evident yet or something seen here now but dismissed as trivial. Of course, when it happens many will claim it couldn't been foreseen.
Remember, even healthy markets can have bear periods. Markets that plunge are sick markets to begin with. It doesn't really matter what the catalyst is that sends them plunging. They were headed that way regardless.