Writing a novel comes to mind,
@Hussar . They can be about wildly different things, from “Little Women” to “Perdido Street Station”. They can be just about any length. And so on.
But their crafting is a process. It requires imagination and then the actual work of writing, and related things like editing. No writers will follow all the same processes or use exactly the same tools, but the bones are the same.
In the end, they’ve written a novel. “Little Women” is nothing at all like “Perdido Street Station”, but yet they are both novels.
But, there is a difference here. We can look at the novel, point to similarities in structure and fairly easily discuss the difference between the novel form, and say, short story or sonnet. Now, fair enough, at what point does a novelette become a novel is a bit of semantic navel gazing that serves very little purpose. But, as an art form, we can fairly confidently state that X is a novel. At least, we can do so for the stuff that falls in the middle of the form. Around the edges, things get a bit fuzzier, but, that's fine.
So, no, novels cannot be "of any length". And, since novels generally follow similar narrative structures, we can fairly often point to this or that work and call it a novel.
But, apparently, something that has no mechanically mandated end point is identical in form to something that ALWAYS has a mechanically mandated end point.

Something where the players are given a (probably fairly short) list of rules approved actions at every given point of the game is no different than something where the players have the option of taking any action and the rules consist of a series of if/then resolutions. In a standard game, the rules tell me, at every point in that game, what I can and cannot do. Sure, I can choose to throw hard or soft, but, at no point can I choose NOT to throw in Cricket. I cannot walk down to the other end, past the batter, and kick over the wicket, can I? If I'm the pitcher (bowler?), I MUST throw the ball, down that line. I have no other options.
There are very, very few points of must in an RPG. At any point in time, the player can choose pretty much any action imaginable, and the rules will simply tell you how to adjudicate that action. Yes, there is that loop of play - RPG's ARE games after all. No one is denying that RPG's are games. And, all games share that - turn taking of some manner. But, where the difference, for me anyway, lies is that the rules in an RPG don't tell you what to do. They tell you how to adjudicate, but, that's about it. It would be like a game of baseball where there are no rules whatsoever, except the strike zone rule. Everything else the two (or more) teams make up on the spot. And what these two teams make up will be idiosyncratic to those two teams and virtually impossible to recreate by any other two teams. Even the size of the teams might vary. Number of bases are up to the two teams. Distances, size of ball, size and type of bat, etc. All would be created by the players. The only point of similarity between two different games is the adjudication of the strike zone.
THAT'S what RPG's are. A list of adjudications without any prescriptive rules that the players then use to create a shared narrative.