Because we should strive to improve discussions and pull them up to a higher level if possible. Especially on HN, where the comment section is actually useful instead of a cesspool of empty arguments and insults.
But saying "This line is a low-effort, cliché dismissal that does not engage the statement in a meaningful way" does none of the things you're hoping for, either.
If I downvote a comment, how is the author to know why? If I want to promote thoughtful discussion on HN, downvoting without explaining why does absolutely nothing to serve that goal.
