" he nevr said what size the tank was" vs. "the poster of the question never said what size the tank was" Is one reason. And I think our society has programmed us to use gender as a form of respect. Say Im in chat talking about a posters tank pics. "Hey did you see Ms. bubbles beautiful tank?" "that person must be a good fishkeeper" sounds wrong when compaired to "yea he really knows what he's doing" (ok I just guessed at he since you didnt specify 
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: . (This happened recently)