Because it's different to different people.
Yes, yes it is. I don't think D&D is about kicking the door, killing things, taking their stuff. I don't think D&D is about classes, or magic, or experience. I don't think it's about Magic Missile, Fireball, and Mordenkainen's Magnificent Mansion. I don't think it's about mapping out a dungeon, unlocking doors, and making saving throws against poison traps. I don't think it's about spamming a cure light wounds wand, or stocking raise dead scrolls.
For me, over the years, D&D has been about story and adventure. It's about a world, a campaign, heroes, villains, authority figures, kingdoms, guilds, cults, commerce, wars, raids, magical events, divine interventions, mysteries, and a whole lot more, than what's written in "D&D rules".
"D&D rules" are simply a tool for conflict resolution. Trying to make them into anything more, is doing injustice to the creativity of the DM and the players.
Campaign worlds could have more specific rules. There could be Greyhawk Wizards, Forgotten Realms Harpers, Dark Sun Gladiators, Eberron Artificers, and so on. But these could be layered on top of a good core medieval fantasy rule set.
Make the core system flexible enough, that when a campaign world is published, the first thing in the campaign source would be the set of "house rules" used for that campaign setting. In Dark Sun, there would be a rule that either disallows or transforms divine classes into something else (like a cleric could be converted to a psionic lay healer), a rule that sets the daily healing available (due to the dangerous nature of the world, I imagine each world would have its own setting for this), a defiling mechanic, and so on.
I'd rather see most of the flavor contained within campaign material, than core rules. Rules do affect flavor, whether we want it to or not. But if the core system has rules for Vancian magic, rules for Power Points, and rules for AEDU, then a campaign can set its tone, by choosing one of these. I could say I'm running a high magic world, so I use power point system, and everyone gets triple power points, and I could make a rule that encourages multiclassing, by giving some random number of starting power points. A world that runs on these rules is sure to feel different than a world that runs on Vancian magic with expensive material components for all magic.
I'm probably rambling too much at this point, but I'll re-iterate my primary point, "D&D rules" are simply a tool for conflict resolution. D&D, is much much more than its rules.
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