Because discussion on most message boards tends to be conducted more like discussion while leaning against a bar: it's informal and conversational. The next time you're involved in an informal conversation, make note of how many sentences are left dangling, usually left with some sort of implied conclusion.
Unlike conversation in a bar however, we don't have body language, facial expressions and voice inflection to help us accurately convey or interpret that dangling implication. So...
Either that, or like others have suggested, we're just dolts trying to mask our inability to complete a coherent thought...