This is the most frustrating part of Scrabble to me. I consider my vocabulary to be pretty good. But frequently when I try to use an even slightly-esoteric word in scrabble, it turns out to not be in the official dictionary. And conversely, the official dictionary is full of words I've never heard or seen used by real people and often has some definition like "18th century slang for <word>". I then lose to people who know all these weird words (especially the very short ones, which they've memorized) and who admit they have never heard these words outside a scrabble context either.
In short, the English the players speak and the English defined by the official dictionary are different enough languages that it seems only useful as a way to learn "Scrabble English" and not modern English.