It is not a desire to kill a thread, it is that many posters do not look at the date of the last few posts on the thread and the start date. Asking a question or making a suggestion on a several year old thread is not useful to the forums. I often see someone post, "You can use XYZ to do ABC" on a My first knife" thread that is several years old. My guess is that the knife is long done with by then. On those type of necroposts, the mods usually close the thread.
Some threads are of universal value and contain basic information that is good for all time - these get resurrected regularly. This usually happens during the two-three week period after school lets out and during Christmas break. If the thread content is still current, then they usually get nothing more than a note from the mods that it is an old thread.
Posting new info on the subject is always of value. Posting "That's really a nice knife" on a six year old Nick Wheeler "How To" thread is pointless.
In some cases, the information in an older thread is not currently accurate, or the OP has changed his methods. In these cases, the mods ( often at the OP request) close the threads to keep conflicting information down. Years ago I may have said that I heat 10XX steel to 14XXF and hold for XXmin. I may now use different parameters, equipment, or have better grade steel. Thus, someone resurrecting a thread with the old numbers will give readers the impression that those are good numbers...which may not be the case anymore.
In cases like Eric's press build, these type threads get resurrected several times. Sometimes by the OP with updates, and sometimes by a new member doing a search. The person doing a search may not realize that Eric isn't following the thread like he was three years ago. If you know the thread is old and have a question, sending him an email will get a better and quicker response.
Advice to all users before posting, especially new ones, is to look at the OP date and the last few posts to see how old a thread is. This is especially important if you found the thread in a search.