This question already has an answer here:
The scenario is a user (often new user) posts a question on the site, some person(s) take the time to write a solution to the problem, the OP leaves a comment thanking the answer poster for solving their problem and moves on without ever accepting the answer. Often not revisiting the site for many months. It can be really annoying to not be rewarded for our efforts contributing to the community, and is accepted as 'just a fact of life on SO', but should it?
We vote to close questions and can flag questions for attention by moderators, so can a flag be used in the same way to alert moderators to an answer that 'should be accepted'. This would only be in the case where there are no other accepted answers and the OP has left a comment along the lines of 'thanks this worked perfectly for me', etc. Not being awarded the rep points when someone has left a comment declaring an answer as the one that worked for them is quite frustrating.
Please note that I am not suggesting moderators be asked to judge the quality of an answer such as here.