Usally when that happens, it's just a new user who doesn't know what they are doing. They either learn that they are doing a bad thing and stop or temporary get banned....
Plus, if the thread automaticly got locked, no one could explain to them that what they are doing isn't good, and they'll do it more, causing more necros
Oh... I see what you're saying...
Idk, I don't think it's too serious to do that yet, it's not like we see groups of people gathering together for the sole purpose of necroing threads.
And if they say, necropost eleven months after the previous post...