If memory serves, at one point there was a note on the page saying "This is a pretty common trope, so please only list examples where it occurs so often that the work is particularly known for it." This may be a part of why there's more detail in entries here than in a lot of other pages.
Give it time. Somebody who's sufficiently Bugged by those to actually start a page about it (slightly more involved than just adding to one that already exists, after all) is bound to come along someday...
Why in God's name do writers/animators still insist on having people gagged with a simple cloth over their mouths even when there's no way it would actually work in real life and everyone knows it?
Because it's easier to draw than a cleave-gag and if they drew a bit-gag, bite-gag or ball-gag it would look ridiculously fetishy?
I'm not sure that's exactly common knowledge.
It isn't. "And everyone knows it" isn't really true. I think the OP is seriously overestimating just how many people end up getting gagged at all in the course of their lives. The only experience the majority of people are going to have with such gags is watching cartoons or TV shows.
I imagine it depends on the context. A cloth over the mouth would indeed stop someone from talking articulately, especially if they're panicked and if it only needs to last for a few minutes (that is, too little time for someone to worm their way out of the gag). And it's quicker to apply, especially when improvising, than other kinds. Try it yourself and see. It won't stop them from making loud noises, but I don't know what kind of a gag would be good for that. So the use of a cloth over the mouth may be implausible and a sign of lazy writers in a BDSM setting, but in some kind of kidnapping scenario or something it might just be the quickest option and good enough for the kidnappers' purposes.
While it is easier by narrative context to write the guy/girl gagged with a simple cloth (the kidnappers might not have access to/have thought to bring anything else), the gag would actually be effective if you added a piece of cloth, stuffed into the captives' mouth, and then with the gag tied over their mouth then. That blocks the voice. A couple anime do it, but its more rare in media involving this trope than it should be. Another good use is wrapping multiple strips of tape over the mouth.
In a live-action production, the answer is often that it's just more comfortable for the actor/actress involved. It's the same reason why you'll often see a character get gagged with duct tape but later the tape gets peeled off smoothly and easily, almost as if they neutralized most of the glue before they put it on (which is exactly what they did).
In many series nowadays, kidnapping victims don't get tied up. They just get left in a room with bad guys with weapons, thinking it makes no sense to tie them up to make them incapable of escaping or doing anything else. It strikes me as a better idea to keep your captive bound so they don't do anything, but just keeping someone in a room doesn't exactly work, from where I stand.
Well, just as a practical filmmaking matter, it's probably easier on the actors to just have them sit in a chair and look scared then to bother with the prop knots and all that.
Easier, i agree. But even from a non-fetish point of view, it's one of those things that just looks lazy or like They Just Didn't Care.