Oh?
D&D Starter Set. I think we can say it is definitely D&D, an RPG. In the box are the rules, pregenerated characters, an adventure, stats for every monster, every spell, every magic item.
How is this not directly playing the RPG?
Ooo. I hadn't thought of that. Good point. Although, to be fair, the Starter Sets aren't what you generally play more than once with any given group. It's a learning tool. Most groups, I think anyway, will take what they've learned in the Starter Set and then create their own campaigns, and thus, their own game.
We are playing different instances of games, surely. If I am playing poker at one table, and you at another table, we are not playing in the same instance, which we'd colloquially say you are in one game, and I in another game. But, that's us beign vague with language. Different instances, but same game.
Sure, true. But, presuming we're both playing the same variant of poker, there's zero difference in how the game is played. The rules say, deal the cards, go around the table for betting, possibly draw new cards, and then resolve the hand. We don't use the rules for poker to play Catan.
OTOH, we do use the rules for an RPG to play many different games. Even under the same umbrella, say, Call of Cthulhu, where each campaign will likely share a number of similarities - i.e. some unnamable horror is going to drive us insane

- the specific games will be very different, table to table. Between house rules, rulings by the game master, and the set up of the campaign, there will be more significant differences between two tables playing Cthulhu than two tables playing Texas Hold'em.
But, by and large, if I am playing my D&D game over here, and then I go visit a friend in a different city, and they ask me to play at their table... my character will fit in just fine. I can play their game pretty seemlessly, in the rules sense. It is like, when I took a vacation to Rome, I was in a different setting I knew very little about, but life was still life, right?
Really? Your character was a cleric for, say, Forgotten Realms. The other table is set in Dark Sun. That other table over there is playing Pre-War of the Lance Dragonlance. This other table is playing Eberron. Your cleric of Tyr cannot actually be played in those campaigns. Rules wise, you actually cannot play in those campaigns.
My current Primeval Thule campaign bans all PC's from classes that have cantrips. Most of the PHB classes cannot be played in my campaign. Am I really playing the same game as the other campaign I play in which is set in Forgotten Realms playing Storm King's Thunder?
Kinda sorta in the sense that the resolution mechanics for both games use 5e rules. But, in a very real sense, those are two pretty distinct games. They have just been created using the same base RPG ruleset though.
Some live action role playing games have this as an aspect - I may play with my local group most of the time, but if I visit another city, I can step into their games, with my current character and even have the XP I earn carry from one to another.
So long as they are inter-operable, in a rules sense, I don't see as they are separate games, really.
And, fair enough. Adventurer's League would certainly come under this rubric. But, by the same token, you cannot play any character that draws from more than one extra source in AL. Many home game characters cannot be played in AL. Nor can any character made using the Unearthed Arcana material.
There are many bars to inter-operability. If being able to port your character from one campaign to another is the baseline here, then many people are actually playing different games since you cannot actually port characters between campaigns. Some you can, sure, but, there are many that your cannot.
I made a serious mistake here though. I thought I was being cute with making a click-bait title for the thread. My bad. It was meant tongue in cheek, but, it fell totally flat.
My point for all of this is to be able to draw the distinction between RPG's and other kinds of games. I really wasn't trying to prove that RPG's aren't games. They are games of course. But, the distinction between RPG's and other games is that there is generally a second step that has to be taken before play can begin - you have to create the campaign in an RPG which is distinct from other games where you lack that second step. That's why I'm saying that the campaign is the game and the RPG is the game creation, and running, engine. It's like the Quake engine. Sure, you can just play Quake. But, you can use the Quake engine to make so many other games that really aren't Quake at all.