This is because some people have discovered that it's a great way to emotionally manipulate others into upvoting that comment out of guilt - the structure of this kind of comment is designed to bypass the logical reasoning centers of the brain and cause an emotional reaction. (I saw another comment a few weeks ago that had more detail on this, but I have no hope of finding it without my exobrain) It's also just barely far enough away from the "Please don't sneer, including at the rest of the community." in the guidelines that some people can justify doing it.
I automatically downvote comments of this form whenever I see them. Comments should be written in such a way that encourages curious and thoughtful conversation and not emotional manipulation.
