You know, I have always wondered why cos-play tags like yurishiro ginko (cosplay) are not a part of character tags... can anyone explain that to me?

I'd say it's a hold over from past ideas of what should and shouldn't be labeled a character tag. There has been some recent arguments on increasing what gets classified as a character tag, although I don't recall if this specifically has been argued over with the various <character>_(cosplay) tags, but I believe it has been mentioned.

One thing is that in the past there were those who relied on the chartags:<number> searches to find images with a specific number of characters (people) in them. Including tags that weren't purely a character (person) and classifying them as a character tag of course would throw off their search results, so there was a lot of push to remove things that would muddle with the results. So for example a person remembers an image of say Ayanami Rei and Souryuu Asuka Langley and their Eva units. To narrow it down they do a chartags:2 Ayanami_Rei Souryuu_Asuka_Langley mecha search, and they'd have expected it turn up the image that they wanted. Doesn't quite work though, as Eva 00 and Eva 02 are character tags, so the image they were looking for actually has 4 character tags on it (and maybe even 5 due to the presence of a Shikinami Asuka Langley tag). To make sure that search works the way they expect it though they'd have turned the Eva 00 and Eva 02 tags into general tags, stating something like "as they're not a person, they're not a character" or similar as the justification. It was the way the site operated for a good while.

The over reliance on the chartags search wasn't a good idea though, as even with trying to push out certain things from getting categorized as character tags you still ended up with oddballs that would throw off the search results regardless. Anything tagged Shikinami Asuka Langley for example ends up with 2 characters tags, as that tag implicates Souryuu Asuka Langley. Eventually there was argument on it on the forum (of which I was one of, if not the one, who made a huge stink over it, as I recall) and the way we operated changed because of it, at least in regards to whether the chartags search should be used to build policy on. The aftermath though hasn't really been hammered out, and I'm sure whatever line that gets drawn will leave someone somewhere dissatisfied with it.