Black, seeing your point, there are places where people spit at the pavements, there are places where they generally don't.
It's a matter of administration.
I agree on the importance of administration. The problem is that the lack of a good forum search engine has consequences that reach far beyond just user inconvenience. If a user can't locate a prior unclosed thread on his topic, he'll start a new one. If a mod sees the new thread, he can't very well close it, knowing that whatever earlier threads which exist may be unreachable by forum search and closing the new one will leave the poster hanging in air.
On the other hand, if a mod leaves the new thread alone, any old thread(s) can greatly age without being kept current. But sooner or later, somebody will tumble upon those old threads somehow and resurrect them (there are other search engines and various reference links besides the forum search engine, of course). Now what is the mod to do in the face of some of the obvious necro-posting?
IMO, the result is a collage of at-times-confusing mod actions or rulings, wherein they are trying to do their best with what they have in situations ultimately made much more difficult because of an unreliable or unsatisfactory forum search engine.
And indeed, as @james438 suggests, it would be useful to some extent if mods would at least post their reason(s) when closing a thread. Otherwise, in some cases, users are left to fret and puzzle at the 'why' of thread closings.