As I was writing about the assumption of a long term campaign in 1st edition, it occurred to me (at least partly) why that is. The game is about the world, not the characters. That's why Gygax keeps going on about "the milieu," that's why the game is supposed to last as long as possible. Because the point is the world that everyone is building.
This is a huge contrast from the sort of game that I usually run, and learned how to run from the 3rd Edition DMG and places like Gnome Stew. The game is about the characters; when they end, when they die or their adventures come to a satisfactory conclusion, the game ends, and you start a new one.
And on a slight tangent, to me, character death makes a lot more sense in a game of the prior variety than the latter. If the point of the game is the world, then one character dying just means it's the end of his particular story, and the chance to add another dimension to the setting. In a game that's about the continuing adventures of one specific group of people, there are probably other ways for them to fail, without having to deal with a character-sized hole in the campaign. Which can work, and be interesting, depending on how the characters are related and what they're doing, but it's a lot more disruptive.