Note: This answer refers to an earlier version of Gil Kalai's answer and does not really make sense for the current version. I preserved it "as is" to keep the developement of opinions visible; as written in an old comment, I essentially agree with the current version of his answer.
The points made by Gil Kalai seem very good, but then they seem based on a wide spread misconception, namely that answering a question as is and closing it forever are the only possibilities.
Yes, of course, let us be charitable in reading question, but why not confirm before answering via a quick comment conversation whether ones charitable reading even is what the questioner was after?
Fine, good answers perhaps make a good question, but why not still trying to improve the question in the process in addition? Won't this make the package still better?
So why is this done so relatively little?
Perhaps because it is a lot of work without much gratification. By contrast writing some high-level essay vaguely related to a couple of sentence the so-called 'question' is fun. One can write on a subject one likes, show-off a bit ones knowledge and typically such general interest material is very well received.
Don't get me wrong I did/do this on occasion, too. So, if you (here I mean an abstract 'you') do it sometimes, too, I won't blame you. But at least don't fool yourself into thinking you are most helpful in doing so.
The actually helpful thing to do, and better for the quality of the site, is to work together with the questioner to figure out first what they are actually after and to answer only then.