Simple question, title says it all.
For example I have a few posts where the answers that are accepted are highly voted up by the community, which indicates they are, based on my questions, vastly right.
Now I am not your copy, paste programmer who lives on SO, but I do use SO for understanding issues and problems and seeing new ideas and solutions to existing issues that I cannot figure out.
Lets take an example post, call it "How to do x in x language"
The question might have an answer with a vote of 100+, while other answers that still come in years later get instantly downvoted. Why does stack allow people to continue answering after a question has accepted what they deem to be the right answer based off the info provided, the votes and maybe the example solution (keyword: example).
I could see if there was no accepted answer, or every answer was below a certain threshhold in terms of voting. But questions after 6 months, regardless of accepted answer or not, should be closed - like GitHub.