Well, they don't have to, but at some odd or even-numbered anniversaries of a well-selling and popular series, it would have been nice to
at least have something exciting to remember the anniversary for. Key years has been a release of a video game, and when we actually did get a game this year, it isn't what the fans were expecting or wanted. Currently, Capcom might not have something else in the plans (or they are hiding something big in the works), but they do gather up the characters from all the sub series and put them together into a few other things, like source books or art books, and sometimes even music CDs, which has been released ever so often.
And then we have the awesome Project X Zone here, which is the main subject of this topic.