Technically, you're right: comments are supposed to be used for feedback on the question, not for suggesting answers. And I know some SE sites take a hard line on 'answers as comments' and tend to delete them on sight, directing the commenters to the answer box instead.
But this is one of those network-wide 'policies' which is more than a little malleable from site to site, and I suggest that we discourage but not delete answer-like comments.
Why not to delete them?
- Deleting them would be removing potentially useful content from the site at very little gain. Surely removing helpful content goes against what SE is supposed to be all about.
- Not deleting them enables us to ping the commenters suggesting they leave an answer instead, and enables them to come back and find the info later to jump-start their answer. So by not deleting such comments, we actually increase the chance of them turning into answers.
- Often what would be a bad answer is still a useful comment: e.g. an unelaborated link to a source of further reading, or an unsupported possible explanation of the issue being asked about.
A common issue on SE is the occasional disconnect between following the letter of the law and using common sense to do what's actually most useful in practice. I've seen various places where people were calling for some action on the grounds of a literal interpretation of something in the help centre, and had to be told e.g. "if it's not causing any problems for the site, don't worry about it" or "look at the big picture of what we're all actually here for, rather than focusing on one particular aspect of one particular policy". This, in my view, is one of those cases: we shouldn't be removing useful information from the site if it's not causing any problems.
Counterarguments would be that comments can't be downvoted (but the voting on answers here isn't always reflective of quality anyway) and that we really need more answers (but deleting answers-as-comments wouldn't achieve that, and in fact would probably be counterproductive for the reasons in the 2nd bullet point above).
How to combat them?
Of course, this isn't to say that we should welcome or encourage people posting answers as comments. As mentioned above, we should respond to such comments asking the commenter to turn them into an answer. If they don't want to or don't respond for a long time, then it might be acceptable to turn someone else's comment into an answer yourself, preferably with attribution to them for the original idea. Depending on how much extra work you put into the answer compared to the original comment, you might want to make it CW, but there's never any obligation to do so.
I've done this a few times here on Lit. Usually I try to put some more effort/research into the answer, so that I don't feel like I'm simply stealing someone else's response. Sometimes the original comment is deleted (probably by the commenter themselves) after the answer is posted.