This, however, is untrue!
The argument that comics would be a literary genre is about as valid as claiming that film is a literary genre. Film is clearly a different medium from literature and while both literature and film can be, and most often are, used for storytelling (i.e., for narrative purposes), they are also different in how they can and do achieve this.
Comics, of course, also operate differently than literature. It is quite clearly more visual in nature than regular literature, which is fundamentally conceptual in that we have to imagine what we as readers "see" before our inner eyes, guided by the literary text itself. Yet, I also find Will Eisner's comment on comics being "movies on paper" to be an equal disservice.to the medium. Sure, the argument could be made that comics are, in some sense, closer as a medium to film than to literature (at any rate, it is at least closer to film than literature is), but even then we are missing things, like film being an audiovisual medium, for instance (whereas comics are merely visual).
Most importantly though, I would argue for the following distinction:
Film is a temporal medium. It shows the viewer a sequence of images all placed, as it were, in the same space yet sequenced in time. As such, a film unlike say a novel actually has a set time for its own consumption (sure we can play with pauses and fast forwarding these days if we want to, but if we just want to watch the film, it is a certain number of minutes).
Comics, however, are a spatial medium. Like film it is based on visual sequential storytelling, but unlike film, the sequence is not set in time but in space. Each frame holds a specific space on a single page, each page holds a specific place in the comicbook or TPB, and the sequences of frames on each page together with the sequences of pages forms the narrative, is the narrative.
I actually found a good example to illustrate this difference when I read the first Angel After the Fall collection. Now don't get me wrong, I really enjoyed this comic, but in the extra material was an included script and I reacted to one thing in particular. Writer Brian Lynch indicates time and again that he builds up each page as a scene thinking very filmically, i.e. he sets each page up to hold a surprise reveal at the end (which will work as a sort of cliffhanger and keep the audience's interest). What struck me, however, was that this did not quite come across in the finished product. In fact, it could not. See, in film, temporal as it is, the end of the scene will not be known until we reach that particular point in time. In a comic, however, anything that I can see by a glance at the end of the page or even at the opposing page, will not come as a full on surprise for me when I reach that place. Rather, I will read the whole scene being fully (or at the very least partially) aware of what the reveal will be. For the surprise to function in a comic, the final panel on the page must instead be the cliffhanger setting up the reveal and the reveal itself needs to be in the first panel on the next page (letting the scene end there, as it were), or even effectively in some cases as a full splash page (the latter of course works particularly well with cliffhanger endings to an issue/chapter, etc.).
Another rather clear cut example, which differentiates comics as a medium, this time from literature, is a sequence from Gaiman and McKean's early masterpiece, Violent Cases. Now, while comics can be "silent" in that they only require sequential art to tell the story (or even make their argument), the medium certainly allows for and commonly uses written words as well (which is perhaps one of the reasons for wanting to view it as a literary genre). However, in Violent Cases, where the narrator is telling a story about a childhood meeting with an osteopath (who was supposedly the osteopath of Al Capone), there is a sequence which could not effectively be translated to written language (which, of course, is the domain of literature). The sequence is basically four panels (see included image) inset at the top of a what is really a full page panel. The part of the story that is being narrated is actually the osteopath relating some of his past in a series of captions, while the images we get are basically a combination of that story as seen or imagined by the osteopath and by the narrator's child self. In particular, the fourth caption and the second panel both verbally and visually introduce O'Banion, a man whom we learn in the following caption "started saying that stuff about Sicilians" (this caption is set in the third panel showing three blazing gun nozzles). The same caption then continues to state that "Al and Johnny Torrio had to have him rubbed out." All of which follow the logic of the blazing gun nozzles and the gangster language. However, the final of the four inset top panels is a repetition of the second panel (i.e. O'Banion's face looking back over his shoulder), only in this panel it is smeared as if someone has started erasing the image, and it sports the caption "(Rubbed out?)", a question from the narrator's child self. The obvious effect is the double meaning of the phrase "rubbed out" and the child's difficulty in comprehending what the story the osteopath is telling means. However, any attempt to capture this double meaning in written language quite simply becomes analytical, and in some sense both less artistic and less eloquent (albeit most likely more verbose). In effect, it shows a strength of the comics medium where any attempt to adapt the piece into a short story or a novel would entail what adaptation theorist Brian McFarlane talks about as adaptation proper (as opposed to a direct transfer); that is, finding a suitable medium equivalent to that medium specific element in the source text.
I have now shown two examples differentiating comics as a medium from both the medium of literature and the medium of film, respectively. So why does the misconception linger? Most likely, it has to do with matters of prestige. The medium of film went through a phase early in its career, where it in some sense attempted to be literature, because of literature being an older, tried and more prestitigous medium and domain. But film gained its own prestige and didn't look back... well, at least not in the sense that it wanted to be literary (now, and for most of its existence, it is merely happy to adapt literature, to use already tried source material as a basis for what is still the most expensive artistic medium around. But I'm sure we'll return to such matters at another time).
Comics, however, despite being roughly as old as film, if not even older, depending on how we define comics as a medium (Scott McCloud's definition in Understanding Comics: The Invisible Art, for instance, suggests that something like the Bayeux Tapestry could arguably be seen as an early comic), has simply not managed to gain the same type of prestige as film. This sadly leads to a lot of comics enthusiasts trying to lean on the prestige of literature by claiming comics as a literary genre, something of literary value, just to prove the value of the comics themselves (and yes, I have in earlier days fallen into that trap myself, I'll admit). The trap here, however, lies in the fact that, while we can argue the value of certain stories told and the telling thereof, to simply compare a piece of comics work to a literary work in literary terms, is quite frankly doing comics a disservice. They cannot do what literature can, or perhaps more accurately, they cannot do it in the same way, just as literature cannot function the same way as comics. For a story to move from one to the other requires adaptation, and adaptation proper at that, because there are medial differences to overcome in that particular translation.