I researched this for a whole two minutes, so correct me if I am wrong.
Those items on those boorus are tagged by a dedicated enthusiast doing the tagging for many, many items and usually in an environment with a rather limited and standardized local flavor of the usual tags seen on similar boorus. Not only is the tag pool small, limited or not, the method of tagging is kinda by some formula.
The contrast on itch is this: there is like 100k persons tagging 10 items. Each with different views and opinions about word and tag meaning. And no guideline on the method how to categorize the item and putting importance on different aspects of the item. They just freely associate their item with tags and have perspective bias, because they created the item and also because they might select tags they think will make the item more popular, while an enthusiast would emphasize different aspects of the item.
I did not understand your first sentence, because I do not know to whom the "they"s referenced and what might not be done. If you were talking about steam, you can suggest tags for games. It reads "Popular user-defined tags for this product:" and shows a short list of tags you can expand to up to 20 tags where you can suggest tags yourself (from a fixed pool of tags).