Tags are placed to let people know what theme(s) the game has -- War, history, zombie, quiz, etc. They do not serve the function of a category, which is more broader -- Fantasy, horror, etc. If you are looking for games under the same tag, your best bests are to use the categories to find them (a fantasy war story is most likely under fantasy for example) or input keywords into the searchbox.
Ultimately, there are so many tags, if tags served the purpose of a category it just splits up the games unnecessarily.
Hopefully this answers your question.