I have been going through the unanswered questions list of this SE and was surprised to see how many of the questions were actually answered in the comments. The commentator then virtually abandons the question and it kindof stays forever unsolved. Why do they not simply post it as an answer?
Alright yes, maybe they wanted it to be more detailed. So would it be a good practice to take one these perfectly good comments (maybe add a point or two) and post it as your own answer?
It is a bad practice to write answers in comments. It's discouraged. Many users fail to understand the difference between a comment and an answer. Some think comments are for answers without citations. Some are lazy to write an answer due to their own reasons. You can use answers posted in comments, add some references to back up, expand it to a detailed answer. This really helps the answered stats. If you are just reproducing them, add attribution to author of the comment. If you are using information, then rephrase it in your own words.
Example: Hamsa Sandesha - a real incident or a fictional work?, my answer and comment underneath the question. I have answered some questions which are answered in comments and flagged the comment as "It's No longer needed". Flags were also approved.
Note of caution: As you do not have enough reputation to look at deleted answers, here is only thing you can miss. Sometimes moderators convert answers to comments. So, there could be a chance of posting the same deleted answer again. Hence, add your information found through research and add it to the answer comment.
Why don't they simply post it as an answer?
Because this site has very strict standards for answers. We have the Back It Up! Rule. Wikipedia- and blog-based answers are discouraged. Writing quality answers takes time. So some people find it more convenient to write unsourced answers as comments. But this isn't a good practice.
So would it be a good practice to take one of these perfectly good comments (maybe add a point or two) and post it as your own answer?
Yes, but you need to follow the guidelines for your answer to be accepted by the community. If the (unsourced) comment is copied and posted as an answer without any additional references, then it can attract a citation banner, a few downvotes and then eventually deleted.
Not trying to discourage you but that's just how the site currently works.
For sources, see What are some online sources to get unabridged Hindu Scriptures or to learn Hinduism?
