Automated Question Parsing
Both Richard and Grace dealt fairly well with the troubles of any sort of automated system which would attempt to auto-answer simple questions.
So I'll ignore that particular problem and assume that we (the collective genius of StackOverflow) have solved that problem.
Parsed Question -> Coherent Answer
The problem with common questions is that they are so very... common. Not only are there lots of root questions (how to multiply) there are enough instances of the variation questions (in js, C#, COBOL) to have each of those question flavours also considered common. Except that a common answer would not be appropriate.
If I ask how long does it take to multply two numbers in python, a general answer isn't that helpful, and a specific C# answer is very unhelpful.
The point here is that even truly common questions will have specific details built in which are less common. So rather than 10 common questions with 10 pre-fab answers, we'd have 10 common questions with hundreds of answers, one for each flavour.
But, this is merely an extension of the parsing problem, although as I've illustrated it is still distinct. We'll once again assume that we (the collective genius of StackOverflow) have solved this problem as well.
Coherent General Answer -> Answer For Me
There is a technique known as Rubber Ducking which basically refers to when telling someone about your problem allows you to solve it yourself.
That is an idealized situation, the most common (but related) one is that when you tell someone, you may solve your own immediate problem, but realize or discover a much larger problem.
In these cases, you'll have follow up questions, your problem isn't "solved" even though the naive case has been.
In these cases, I have a common question that is the root, but there is a much more serious problem existing in the question text. So, the automated answer robot
(we'll call her Compendia) will post a generic response, completely ignoring the warning signs in the question. The OP will add a comment asking for further details after having edited their question, or more likely, simply saying
"It didn't work, now I'm scared. Hold me." -Sincerely, The OP
At this point, there is no one looking at their question. The regulars will have learned to avoid questions that look 'typical' because Compendia will answer them all and so there's no gain in bothering. The original answerer cannot add further information because she's not real people.
Basically, this will lead to a lot of unsatisfactory answers to "common" questions, and since common questions are the most... common... this would end up having a significant detrimental effect on the experience for nearly everyone on the community.
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