I can see previous meta discussions on what should be a comment and what should be an answer, but what I'm wondering is, why are so many users posting answers, sometimes quite in-depth (if compact) answers, as comments? Does anyone have any insight? Should there be more encouragement (not any sort of coercion) to actually put things in answers even if they are succinct yet complete?
Huh, weird. This gets asked often enough in metas, and I was fairly certain it was asked here too, but for the life of me I can't seem to find it. Anyway, here's the wisdom of the ancients on it.
Bottom line is, people develop a certain 'answer barrier', meaning, what they don't consider a high quality answer for whatever reason1 goes in a comment instead because the alternatives would be to either not guide the OP in any way or to post an answer they themselves aren't too satisfied with.
I disagree with some folks thinking this is a way to avoid the possible penalties of a low-quality answer (you can't downvote comments) as these folks are often well-intentioned but indeed, this is not how comments are supposed to work. There's occasionally some friction when folks come to meta to complain about a comment-answer being removed by moderators.
Regarding your last question, I don't recall any recourse sites have come up with other than some guidance in meta posts asking people not to do it or a more strict/systematic removal of comment-answers by site moderators so as to make intolerance against them part of "site culture". Both options are far from ideal, obviously: Meta posts create a temporary hype but they tend to get forgotten over time and there's the friction thing that comes stricter moderation (accusations of censorship and hostility etc. being the most glaring, although not the best examples) and if you did come up with an idea, it might well be worth a discussion or feature-request!
By the way, I ironically (?) started typing this in a comment and then just realized the topic of the post I am comment-answering on . . .
1: The most common reasons are that either the question is close-able and answering off-topic posts is explicitly discouraged, or they don't have the time to verify their claims, or think they don't have the expertise to deal with all the nuances of the question, or they feel the answer is incomplete for any other reason.
I probably get "questioned / criticized" more than most ELL users for making comments that others think should have be posted as formal "Answers".
I don't really see any harm in my approach, and I certainly don't care if there happen to be "site guidelines" advising people not to do this. Within reason, each site is free to honour such guidelines to the extent that the specific user base (and their elected representatives, the mods) see fit.
Just today I posted this comment - which received two upticks, and two comments suggesting I should have posted what I had to say as an answer rather than a comment.
Part of my reason for just commenting was that it takes much longer for me to write an actual answer. And that's not just because (as implied by my "nom de plume" here) I'm very slow at typing - it's also because I'm acutely aware that my first impressions are often mistaken. In the case of the linked comment, that's unquestionably the case from my perspective (the eventual answer was much more accurate than I was).
A related factor here is that if someone like me posts an off-the-cuff reply / reaction to a question, there's every possibility that someone else will be minded to take issue with the substance of whatever I say (as opposed to simply taking issue with me saying it in a comment rather than an answer, which I do sometimes find rather tiresome).
Personally, I suspect that people often start arguing / nitpicking over the finer points of detail raised by a question because they don't agree 100% with some simplistic response given in a comment. To my mind, this can eventually lead to higher quality answers, because two (or several) heads are better than one.
TL;DR: I already feel under enough pressure to answer more and comment less. I'm not asking to be treated less severely than I am at the moment (but I do get irritated sometimes if I think my "relevant" comments have been over-hastily deleted), but please don't ask the mods to be even harsher than they are. So far as I'm concerned, they do a great job, and the balance is currently just about right.