The accepted answer mentions "Math.SE as a repository" and I think this divide is exactly crucial to understanding this behavior. Math.SE, like all SE sites (in theory), simultaneously wants to be a repository of useful Q/A, and a community.
When a user posts a question that is neither bad nor good -- a near-duplicate, something not fully thought out with a simple issue, or etc. -- a typical reaction is to close the question with a vague justification. This is good for the repository, since the question is unlikely to be useful to other visitors. On the other hand, it's bad for the community, since this new user isn't likely to learn what they did wrong from these pro forma closures, they're likely to learn that this place is hostile, and quit. That happens a lot! It's not an edge case!
The other obvious action is to just answer the question. Format it nice, whatever, it's good for your rep, you get internet points, and the asker is happy. This is (sort of) good for the community, because it leaves the asker with a good experience and they're likely to come back. But it's bad for the repository because this question isn't great, it's not likely to have benefits to future searchers, etc., and the question might even get closed later (which is confusing to most non-power-users).
So when somebody understands the trade-off, and doesn't need to farm rep by answering easy questions, they'll often give a quick sketch of an answer in a comment. A "real" answer isn't needed because the question is unlikely to get a lot of attention anyway, and a comment is faster. Additionally, the small text also communicates the smallness of the answer, and functions as a soft admonishment to the asker, like they've used a heavy tool for a light task.
You might not like it. I'm sure people will continue to complain about this, and delete comments, and so on. But nobody has yet resolved the repository vs community issue -- and SE desperately needs to be both -- and this middle action has proven itself to be a useful middle ground between the two "correct" actions.
Now of course this is all assuming the question is neither bad nor good, which is usually what I see in this situation. If you see a question with a comment-only answer that's actually good, please, make a real answer (feel free to cite the comment if you want).