While I understand your point of view on this, there are two possible outcomes:
- The post is deleted, and the comments become irrelevant
- The post passes the LQP review without deletion, which again, make the comments irrelevant but visible.
The post is deleted, and the comments become irrelevant
These are typically new users who post "Link Only" or otherwise low quality answers, and these comments are beneficial in assisting the user to understand what makes a good question. It is preferred that the user fixes their answer prior to it ever hitting the LQP review, which is why the comment is made in the first place.
However, not every user makes revisions to their answers. The people who leave these auto comments are beneficial in the sense that they are trying to prevent the answer from being deleted in the first place. These auto comments leave a detailed explanation on why their post may be deleted, so OP can proactively make changes to prevent that from happening.
The post passes the LQP review without deletion
This can happen for a variety of reasons, such as the question being edited to be make the post a quality answer or the LQP queue being in disagreement with the flag and it passing review.
For starters, the users of the Userscript that is used to post these automatic comments are generally notified in the SOBotics chatroom when the post has been edited. This allows the user who posted this autocomment the ability to re-review the post and determine if the comment should be removed and the flag retracted. Unfortunately, most people do not edit their post after the useful feedback is left, which is why it gets placed in the LQP queue.
If it's determined that the question did pass the review queue1 and these comments still remain, you are free to flag the comment as "No Longer Necessary". While I agree these comments can be redundant, this is usually the fault of the queue reviewers for leaving a comment when one is already made, not of the user flagging the post to begin with (and allowing the poster to make favorable changes).
1 There are many userscripts available on Stackapps that allow you to easily view the timeline on any post. You will be able to see review results and the history of that post.