I don't know where to exactly look for the answer to this question on SE / Meta.SE as my searching has been inconclusive.
Why doesn't Stack Exchange require users to enter a comment as a reason for a down-vote? I know they lose a point for not providing a reason, but if they feel the question isn't of good quality - why don't they request an edit from the author of that post?
The tooltip that pops up when downvoting a post reads:
Please consider adding a comment if you think this post can be improved.
Not every post can be improved. Some posts are simply bad and deserve downvotes. Others are simply inaccurate, or orthogonal and do not answer the question properly as written, and deserve downvotes. In these cases, the only way to 'improve' the post is to delete it. A comment would not help to improve it. Beyond that, there is an asymmetry of effort and cost involved; Downvoting requires far less effort than commenting, and does not expose the voter to retaliatory downvoting, or the possibility of getting into a protracted argument.
Beyond that, there are other reasons to downvote as well, quoting Shog9 in one of the meta links linked above:
My personal feeling is that votes - up or down - have their primary value as hints to other readers: which questions are worth reading, which answers should i read first, next, at all. And i vote accordingly, doing what the site lets me to adjust the scores of answers to a given question to where they match my own personal opinion of what matters. I won't vote a decent answer to where it scores < 0, but i see no reason to leave it > 1 if a better answer scores lower, because if i were searching out an answer to the question, i'd want to read the most comprehensive answers first. I don't have a hard and fast rule on this though; sometimes i'll vote on every answer to a question, other times i'll only vote for one or two.
Put another way, why do I need to help you improve your sub-par answer if a better one is already there from somebody else? And if yours outscores theirs, well, I'm going to use my votes to improve the sorting so that the next person who finds this question is more likely to see the better answer first.
But above all else, remember not to get too hung up on downvotes; It only takes 1 upvote to overcome 5 of them, and the down-voter pays a price for each one. They're more important as a sorting mechanism for a site than as a way to hurt your reputation.
Because voting is an anonymous action.
Requiring a "reason" simply isn't possible. It would be possible to require that you type something before you can downvote a question or an answer, but that would have a number of side effects, most of which directly oppose the purpose of allowing downvotes in the first place: to demote low-quality questions and answers.
They could require that you leave a comment when downvoting, but there's nothing to stop you from putting "This is the required comment for downvoting" each time. They could apply some sort of filter to screen out "standard" filler comments, but those are easily circumvented. Comments like that could be flagged, but mods and high-rep users already deal with a number of flags, and we don't want to put more work on their plate when it's not clear that it would have a positive impact on the site.
It's not uncommon that a previous user will already have left a comment explaining their downvote; it doesn't seem productive to force a subsequent user who also downvotes to post a similar or identical comment when there's already a system in place to handle that in a more compact fashion (upvoting the existing comment).
The end result of any restraint on downvoting is a decrease in the number of downvotes cast: this is in direct opposition to the more recent direction from the SE folks, including the removal of the rep penalty for downvoting a question. If anything, we'd be more likely to see something that would encourage downvoting, like removing the rep penalty for downvoting answers.
