If you notice,
1. most of them just joined, brand new to the forum.
2. They only make the one post.
3. They are never heard from again.
They are called "bots" and they are traffic generators only, they are not real people.
Now, not all the old post that are rebrought up are bots, some I know are real legit people and they correspond in different threads. I sometimes will "watch" the new person when I see this for a few days. If you click to see their other post, the "bots",or the ones I suspect are "bots", never have any other post, others will be active in several threads.