In a typical game, you read the rules, follow the rules and play the game. Deal 5 cards, have rounds of betting until everyone calls and then show your cards - the rules of Poker pretty much directly line up with what you do when you play Poker. But, if you read the three core D&D 5e books, you can't actually play the game.
Well, a recurring claim made on The Forge is that many RPGs are games with poorly written rulebooks!
If you sit down and read the rules of Dungeon World, or Burning Wheel, or The Dying Earth, you can start playing the game. (One player might have to start a bit earlier - that's the case for DW and TDE, not so much for BW - but that could be compared to the need for someone to shuffle and deal in a card game.)
The same thing is almost true for Over the Edge, Pendragon and Prince Valiant. And now that I thikn about it, a very early game for which it is completely true is Classic Traveller - which doesn't even require the GM to stat earlier, because the patron generation and world generation systems can be used to create fictional content for play as things go along.
But, the rules of poker specifically tell you to deal the cards. The rules of Monopoly specifically tell you how to set up the board and what each player can do each turn. You don't read the rules of Monopoly or Poker and then set up your own board (although you can) or pull out Uno cards. In order to play those games, the game itself defines the setup.
RPG's do not. The game gives a number of guidelines on how to create a game, but, you cannot actually play D&D without first creating a game.
Again, this may be true of D&D (all versions? most of them? Moldvay Basic has pretty clear instructions on how to set up the game), but it isn't true of all RPGs.
The Dying Earth rulebook tells the players what the steps are that one of them (the GM) has to take to kick things off, in its instructions for scenario design. The Dungeon World rulebook gives very clear instructions on how to prepare for and run a first session, and then what to do prior to subsequent sessions. Classic Traveller also has clear instructions for making things happen - remarkably clear given its vintage! (Where it's quite unclear is in instructing players what they actually do in the play of the game once fictional content is generated.)
Every single time you play an RPG, the set up is different and the rules don't dictate any initial conditions.
Every time I play fiver hundred the set up is different (ie the hands and kitty contain different mixes of cards compared to the preivous hands and kitty). The process is the same from hand to hand (shuffle the cards, and then deal them out in a certain pattern) but that's true for RPGs also (identify a starting situation for the PCs using whatever method the rulebook states, and then ask the players what their PCs do). Of course some RPGs don't suggest any very solid method for establishing a starting situation (eg OD&D is pretty weak on this, and the AD&D DMG isn't super-strong either), but that goes back to the "bad rulebook" point.
It's not true that the rules of a RPG don't dictate an initial situation. In most cases they dictate a rather precise initial situation, namely, one in which someone establishes a fiction in which those characters who are the PCs find themselves in (imagined) circumstances that propel them into some sort of action. That action is the play of the game.
Different RPGs have different rules and instructions - express and implied - for who establishes that initial fiction, and for what counts as an imagined circumstance sufficient to propel a fictional character into action, and for what
sort of action those characters should be propelled into (eg in classic D&D, the most salient action is an attempt to recover treasure from a dungeon).
Which is why discussing RPG's becomes so problematic because no two tables is EVER playing the same game.
I think there are two reasons that discusssing RPGs is sometimes hard.
One is related to your point: one group is playing poker, another is sitting around playing solitaire together, but they expect to be able to talk to one another not only about shuffling, or some general features of dealing like how to handle sticky cards, but the
details of dealing in various patterns, and the details of card play as if each group is doing the same thing. And then when someone points out that they're playing different games, each starts insisting that poker, or solitaire, is not a
real card game.
But the second reason is a bit different: the above situation never (literally) comes about, because no card player ever had trouble acknowledging that their are other card games, nor (shoudl they be so inclined) to describing the features (of deal patterns, of play, etc) that distinguish those games.
Whereas for reasons that I don't get, many RPGers are bizarrely reluctant to talk about their games as games that follow various procedures and deploy various techniques, to identify the ones they use which make their game different from someone else's games that uses different techniques, and hence to facilitate coherent discussion across peole playing related by different games.
So whereas the solitaire players will find it pretty easy to pick up tips for dealing with sticky cards from the poker players, without being berated for not betting and for not having one hand per player, in the RPG context it's quite hard to get coherent discussion of points of commonality where advice can be given without encountering totalising arguments that all RPGing must be this one thing that is how that particular person plays.