Why do people get mad when old threads are revived in message boards or forums?

I've seen this a lot on message boards/forums where people learn something or have something new to add to a really old thread. After they post a lot of times theres some jerky person who chimes in and tells them that is was stupid to revive a thread because "that was from over a YEAR ago check the date!", or something else blah blah blah. I don't understand this. How come it is a problem to revive an old thread and especially why is it a problem when there in new information to be shared from it? Isn't the purpose of a message board supposed to have a place where information can be shared? What is up with this dumb message board etiquette where people have to be apologetic for doing things that are normal? Are people stupid?