Answers flat-out shouldn't be posted in comments. Beyond the reasons suggested by mxyzplk — we're not using the site correctly, and everyone gets robbed of a soldified answer — I want to highlight a very deep problem with answers in comments:
Comments have virtually no quality assurance.
When you post an answer, the entire question is bumped for everyone to see. Your answer is big and obvious. It has enormous upvote and downvote buttons. People can review it, point out critical issues you need to address, improve it now or later if/when it's worthwhile doing so, and downvote it if it's wrong. Plus, the question overall gets more activity and attention by virtue of the bump, which is healthy for it.
Comments have none of that. They just silently appear, will probably not get noticed (except by the asker), and only have an upvote button.
So providing an answer in comments, or permitting one, is irresponsible: you subvert the community-checked quality assurance process that validates or refutes your solution. You could provide an answer in comments (we'd delete it, but you can still try) — but maybe this is the time you provide bad/mistaken guidance. Nobody notices to respond to it, asker takes it and leaves, is never seen again, and operates on bad advice. (Good job.) If you're confident you're correct, submit your answer to our QA process and see. If you're not confident, either don't post at all, or post an answer anyway and see if we really do downvote it.
There's some common motivations people have for doing this anyway, and I'd like to address those:
To those deliberately giving up rep: You may be nobly sacrificing your reputation via answers in comments, but please don't do that. You're also sacrificing site quality, which sucks! Please use our answer mechanisms so that we can keep the site high quality, cement your answer for future visitors when it's the correct one, etc.
If you're leaving the comment so someone else can make a better answer: They won't. We need you to do that yourself. People probably just won't notice your comment. (If the question's bright and new, it's probably got the attention of people who have already thought of that.) If it's bad/incorrect guidance, nobody will want to do that, but then we'll have no way to show it's bad guidance.