Futanari *requires* both a pussy *and* a cock. Now, the optional part is the balls, you'll see many different art of some with also called "full-package futanari" or those without. Either one of these is fine and will be called Futa, there won't be complaints there. However, no pussy and it's no longer a futa anymore.
Your post kind of also answers your own question, the other more accurate tags for it only have 6 games. Where the more widely used term has many, however, it's piggybacking off a term more people know and have certain expectations for.