Well, while I agree that it's more common for people in general to be able to come up with ideas than being able to implement them, in teams there is generally room for both.
Coming up with ideas is also a skill. You can have talent for it, sure, but there is a deep set of methodology which goes far beyond intuitite, naive ideation. People who implement things tend to also have limited time to come up with ideas, as a result a team of ideation, design, implementation and testing competences can have room for all sorts of people.
Furthermore, games that end up as ideas or designs can still have value. Maybe they are implemented in the future, in another format or just constitute excellent expansion of the ideators mind. But I agree that it is fun to manifest ideas for a variety of reasons as well. : )