Most competitions (why is it a spelling bee and not a spelling competition?) seem to revolve around a specific skill that stands on its own, and it's rarely about retaining facts that are described in a book. Compared to a physical sport, it's as if you'd remember all the rules but then not play the game. Or compared to a sport like Go or Chess, there is no interaction with an opponent, there is no improvisation or strategy, just data.
Maybe it's an American thing, and it holds it's value because it's a tradition that was recently created (well, relatively recent). It's what most searches seem to suggest... (as well as it being the reason that I don't run in to a 'bee' in other countries)