I don't have to. It's literally THE definition. If you have an issue with it, argue the case with those who defined it.
Again Max we have already hashed over this discussion and it isn't this simple, and we clearly are not going to settle it here. But I've made several responses to the rhetorical and definitional arguments you are using. Problem one: you use the first line from the first definition of literature that crops up on a google search. Problem two, you ignore important qualifiers in the definition. You ignore secondary definitions. You ignore other definitions from different dictionaries and from more long form sources. You focus on the first two words of the first sentence and do so in a very expansive way: "written works". Not only is this vague, the word 'works' to me doesn't merely suggest 'all words on a page' it suggests completed projects. A wouldn't describe a handwritten note on lined notebook paper as a 'written work' for example. But most importantly, if anything written ever, for any purpose, of any quality, is literature, there really isn't much point to the term. The fact is literature is a word that shifts meaning depending on how it is used. In this thread, we have mainly been talking about quality of the words. So that sense of the word seems most appropriate. If you want to use a broad meaning of literature, such as 'written works', in order to make arguments related more to quality of the works, then you are equivocating.
It absolutely does have a point. Once people can accept the facts and understand that anything written is literary, the question stops being, "Is this literary" and becomes, "What level of literary is preferred?" or perhaps, "What is the average level of literary language in RPGs?" It helps clarify things and direct the conversation to the point where it can progress. The OP asks the wrong question.
I don't think it does, because I think this assumption that it is literary is very much in dispute. And I think if we accept this conclusion, then it begins making literary quality a measure of GM and RPG quality (which I don't think it is at all). I would agree you can have a spectrum of 'literariness', but you wouldn't describe something that is at the far end of not being literary as 'literary'.
Oral literature is a thing, which means that oral literary techniques are a thing.
This is a term I am less familiar with, so I don't really have an opinion on it. But I would make the point that in the context of this discussion, this makes for an even more meaningless use of the word. It was expansive and pointless enough to have literature to mean 'written words'. Now you are using it to mean 'any words' (but I see music is on the description of Oral Literature, so perhaps 'any communication' is more apt). Basically if that is your definition, then it is going to happen no matter what, unless we are not communicating during play, so it doesn't matter. If it just automatically becomes literary, what is the point? Are you seriously arguing because you've made a linguistic argument that RPGs must always be literature, that this means anything in terms of porting in literary techniques? Because that argument doesn't get you there. All it does is establish, wrongly I think, that games are always literary. It doesn't tell us anything about what techniques used to make written literature and fictional stories would be effective, necessary or desirable in an RPG. You are literally just playing word games here.