Usually I try to edit the thread poster's post in the merged thread to say why.
I meant in general, not specifically. I meant that you haven’t specified your reasoning for your general approach.
Quite the opposite, I'm afraid. Most forums I've been to have very little moderation and are left with dozens of garbage threads akin to, "WHO'S THE HOTTEST FF GIRL?", "WHAT'S YOUR FAVORITE W/E?", etc. Those types of threads are just there to breed post-whores and usually mark the end of a forums (or at least anything resembling discussion on any real level).
Such boards suffer not from having too many threads but from a lack of posting quality and lacking posting standards.
I'd rather have friendly giant threads anyone can jump into, which, I admit, are probably intimidating to n00bs or w/e…
Plonking everything into the megathreads discourages full exploration and discussion of individual items. The pace of the megathreads is much higher because they cover so much, and it’s much harder to have ongoing or deep exchanges in them because the current’s so swift.
…rather than pages & pages of threads that are too similar to one another…
Better to have pages and pages of threads that run their course and can be revived at the expense of encouraging searching and necromancy than having a few limited or overworked megathreads dominating the forum and limiting discussion and discouraging people from starting new threads.
If there's already a thread for it, why not post in that rather than make an entirely new thread?
You’re oversimplifying. I’ve already pointed out that megathreads are problematic. You still haven’t stated why a greater number of more specific threads is necessarily a bad thing, or why it would be such a great problem with this particular forum that it wouldn’t work given a bit of time and perhaps some mod intervention in the beginning.
That aside, merging threads—which kills them even if the bodies are still there—isn’t the only option. It’s much more constructive to broaden the scopes of overly specific threads or merge several of them than terminate them.