This makes sense from the standpoint of Lit Crit - there are only a certain number of plots and they have been categorized, analyzed, and grouped. Same goes for characters - if you don't believe me check out TVTropes.org. As for the message, SF seems to be especially undiversified in that area. Sure, it does a heck of a job investigating nuanced questions of ethics, morality, theology, philosophy, and the sciences via the quasi-magical deus ex machina of Technology, but the fact that it reuses that deus ex machina so often means that some messages inevitably reappear far more often than we would like as consumers or creators of the genre.
In particular, I'm thinking of the dichotomous Technology Is Evil / Technology Is Good message. It sounds like what it is - works of SF inevitably seem to take the stance (implied or overt) that the technology enabling the situations discussed is either Good or Bad. This pratfall can be, and sometimes is, avoided by artists who take care to show that the enabling technology of their work can be used for Good or Bad by Free Agents, but we are speaking in generalities for now, and generally speaking, SF work can be split into these two camps - a fissure that often runs along the similarly deep divide between utopian and dystopian SF.
That leaves imagery and setting. Imagery is great, but what does it usually rely on? The sweeping vistas, vast stretches of time, and colossal armadas made possible by the SETTING. So if you are looking to liven up your SF, you look for a good setting.
The process of constructing this setting is known, in the parlance of the people who do these things, as worldbuilding - and it is a lot of fun. So fun, in fact, that many artists fail to ever begin their actual narrative and instead get stuck in a kind of obsessive, recursive loop.
Take, for instance, Dinotopia, by James Gurney. This series of books and its accompanying website together construct the universe of an undiscovered island of Earth where humans and intelligent dinosaurs live in harmony. A strange excursion into utopian worldbuilding, the Dinotopian universe is especially notable because most of the books that comprise its canon do not so much center around a narrative as they do around a detailed depiction of this setting. Certainly, a simple plot develops and runs to its conclusion, but one gets the sense after reading these books (and looking at their astoundingly detailed illustrations and diagrams) that the story was merely an atrophied vehicle to drive the exploration of the universe. Indeed, it seems possible that if Gurney had thought he could flaunt convention so completely, he might have eschewed the narrative entirely without significant damage to the power of his art.
Don't go thinking this is a fairly unique phenomenon either: other artists, even communities, have put narrative in the backseat while their encyclopedic dialectics working out the innards of a world that never was go for a drive. Orion's Arm, an online community dedicated to creating an SF universe with transhuman elements as plausibly as possible, is one example of a particularly long-lasting community of worldbuilders.
Even more popular universes are not immune to this sort of fiction. Star Wars features such books as The Illustrated Star Wars Universe and The Star Wars Essential Guide series. While these books use the material which fleshes out the traditional narrative expressions of their universes as a starting point, much of the information contained in these volumes is either assumed, deduced, induced, or entirely fabricated. Jumping onto this trend's bandwagon are books like Fantastic Beasts and Where to Find Them, by Newt Scamander (aka J.K. Rowling), and The Dune Encyclopedia, edited by Dr. Willis E. McNelly. It would seem that if a universe is inviting enough, there is a market made up of people who simply want to explore its niches and unmapped territories, to make it even more "real" in their minds by learning new details. After all, as any good lier knows, the least questioned stories are generally the ones with more consistent details. To achieve verisimilitude, a facsimile must be nuanced. The real world is complex, and so are the best of our fantasies.
I must confess I am not an unbiased witness - even as children my brother and I created our own fantastic universe, which you can view in this slideshow here (be sure to access the speaker's notes by opening it in a new window using the button on the lower left of the player and then clicking Actions. They provide much-needed explanations for the odd illustrations):
The point then, as it seems to be now, was to create a platform for an infinite variety of narratives to take off from in the mind. Thus in its way, the worldbuilding mode is more versatile and free than even the novel or the non-sandbox computer game. These media cannot rove over the countryside of a newly-imagined universe without considerable difficulty, because they are constrained by the rigid train-tracks of a narrative plot that must show only what is relevant to progressing the storyline if it is to maintain proper pacing and keep the reader's interest. Although many of us love the discursive examinations of esoterica found in books like Moby Dick, this sort of sidetracked writing style is often criticized. It appears ill-fit with the narrative it purports to uphold. It might often do better liberated from the shackles of its parasitic narrative structure.
In a round-about way of argumentation, I suppose I have been attempting to point out that this uniquely postmodern mode of fictive artistic expression deserves recognition as a subgenre within SF & Fantasy. Worldbuilding is a natural and valid mode in and of its own right, and offers opportunities to artists that no other present medium seems prepared to provide.