Why: Because it often results in misunderstandings and necessary additional communication if the comment isn’t related to a specific code line or part of a line. You either can quote the related code in your comment (additional time to review) or you get questions from the one processing your comments (additional time until a commit is ready to merge). Additionally, you can’t have multiple comments to one code line yet.
How to solve: I suggest to enable the reviewer to mark a code snippet and have a tooltip (like in Confluence) to write his comment related to the marked code. Also the code snippet marked should be highlighted (as in Confluence).