There is a certain amount of altruism involved in answering a post. After all a lot of people just lurk and benefit and don't contribute. Of course lurking is OK too it is a good way to learn.
If you answer a question posted by someone who only posted once 8 years ago your altruistic instincts may be better applied if you check to see if the original poster is likely to benefit. Unless you are posting to an old thread because you believe the subject is still relevant and you believe others will benefit.
In any event I think the regulars are just making sure the posters are aware of their options.
What I'm going to do, when I remember, when posting to an old thread is to mention that I know the thread is old so folks know it was a conscience choice.