This question already has an answer here:
When someone posts an answer that's really a comment, mods can move it to the comments.
But when someone posts a comment that is essentially a complete answer (and no further answer is likely to be forthcoming) about all we can do to get the answer to be an answer on the question is say "please post that as an answer".
Should mods be given the ability to go the other way, making answers-in-comments into actual answers (simultaneously deleting the comment)?
[Note that this is not the same thing as Add a “convert-comment-to-answer” button which is about commenters making their own comments into answers, nor is it the same as Mark a comment as an answer (which is about askers making comments that answer their questions into answers, and which the previous one should not be marked as a duplicate of since they're asking quite distinct things); it's fairly obvious that those two questions could have very different answers to each other, and both could have very different answers to this question, which is about moderators converting comments to answers. Different questions, requiring different sorts of answers.]
If the original poster of the comment has some strong reason to not want it as an answer they are still able to delete the answer, of course, but the comment would (and generally should) remain deleted in that case.