I think this is a good question to ask. It ties into what I was getting at with a few of my posts both here in this thread as well as ones I've made elsewhere.
For me, my answer was that I was lead to believe McDonald's made pizza -to keep with the same analogy. The vast majority of my rpg experience at one time had been D&D and virtually nothing other than D&D. I was vaguely aware that other games existed, but I mostly assumed that most ways of rolling dice and playing a rpg were pretty much that same. I did have a very brief period of playing Rifts when first introduced to the hobby, but the GM of the game I was in did most of the rolling for players and my time with the game was very brief, so I did not see a lot of how it worked at the time.
So, honestly, my education level concerning games and game design was very poor at that time. Looking back on some old posts I made concerning D&D, I can now admit to myself that I held a lot of opinions about what I thought I liked due to not knowing any better. While some of 4th's changes were things I did not like (some were things I did like too,) they helped me to discover (through being so different from what I knew in 3rd) that there was more to pretending to be an elf or slaying a dragon than simply rolling a d20, and that styles of mechanics might actually change the flavor of the game.
I suppose my personal answer is that for years I had been eating chicken nuggets and thinking they were pizza. I had been told it was pizza. Not ever having pizza, I had no idea that McDonald's chicken nuggets were not pizza. Now that I've had pizza, I am aware that it tastes nothing like chicken nuggets, but there was a time when I would have never known the difference. Had somebody told me I was eating what I thought was pizza the wrong way, I would have argued with them and defended my chicken nuggets as being the real pizza.
It would not surprise me to find that there are others right now who are eating 'chicken nuggets' and believing they are in fact pieces of 'pizza.'
Yes. However, in the pizza analogy, a game system is not a single meal, but a menu or series of meals over some time. If you want pizza, and you go to McDonalds and get chicken nuggets, then you really don't have a leg to stand on. (Try to write a complaint to McDonalds or interested observers that will sound reasonsable.

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But the fair claim for immersion in D&D is not that. Rather, it is that if I'm going to frequent your place all year long, there had better be pizza as an option part of the time. Any given meal, a whole bunch of given meals, with no pizza? No problem. No pizza whatsoever for a whole year? Problem.
My complaint with the (particular, more strident) advocates of "more immersion, more immersion!" all the time in D&D (i.e. the assumption that immersion is the paramount virtue in any RPG, including D&D) is that they aren't lobbying for pizza on the menu. They want to replace pretty much everything else on the menu with varieties of pizza. You can have your hamburger and chicken as toppings, and you can get the shoddy version at McDonalds, the tasty version at a nicer place, and maybe some Asian flavoring at the Japanese steak house. But it will all be pizza. When you cook at home, you can make anything you want, but it will only be made with pizza ingredients.
Guy I knew in high school worked at a very nice pizza place for awhile that let the employees cook and eat as much as they wanted with the food that was there. He said it worked great for awhile. They made some mean omelets for variety. But after two or three months, you got sick of the ingredients. I worked at a nearby hamburger joint. We used to trade them a bag of burgers for pizza, since we were sick of our food.
Then you have the "disassociated" nonsense which is basically folks claiming that you can tell how appropriate a mechanic is for D&D by testing to see whether or not you can fool yourself. A hamburger is bad as a hamburger. But if you can fool yourself into thinking it is pizza, well alright!
The exact, optimal amount of immersion producing mechanics--and lack of immersion destroying mechanics--is a fair point of discussion. And so is, if D&D is truly to remain rather middle of the road, support for other styles of play. You merely can't get very far in that discussion if some of the people involved will not acknowledge the inherent limits of going down the middle. One of them is that "more pizza" is not automatically a good thing. I know that is hard to believe. There was a time when I wouldn't have believed it.
And yes again, wider experience in games (and food) will make one a more discerning critic of the limits of a particular game. Lack of this experience is not automatically crippling, but it runs the risk of leading one into very shallow argument that try to turn hamburgers into fake pizzas.