I can speculate as to why people do this, but given my inability to escape the behaviour, I clearly don't understand it very well.
To a certain extent, I'm also surprised that it happens on Less Wrong, which I would credit with above-average reading comprehension skills. Answering the question you want to answer, rather than the question that was asked, is something I'd expect less of here.
That's fair.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.