Lore accumulates. It accumulates fast. A dungeon grows into a wilderness which grows into a campaign world. When I was 14, and I had just started a new AD&D campaign, I drew a circle in the middle of a piece of paper and said to the players: 'This is an inland sea. The dwarves live to the north-east and the elves live to the south-east and the humans live everywhere else.' By the time I was 18 I had written hundreds of pages of information on the geography and history and races and religions of the enormous fantasy world which now sprawled out in every direction from that original circle-on-a-map.
From Stephen King's On Writing:
Stories are relics, part of an undiscovered pre-existing world. The writer’s job is to use the tools in his or her toolbox to get as much of each one out of the ground intact as possible. Sometimes the fossil you uncover is small; a seashell. Sometimes it’s enormous, a Tyrannosaurus Rex with all those gigantic ribs and grinning teeth.
From Tolkien's Tree and Leaf:
The Lord of the Rings was beginning to unroll itself and to unfold prospects of labor and exploration in yet unknown country as daunting to me as to the hobbits. . . . I had then no more notion than they had of what had become of Gandalf or who Strider was; and I had begun to despair of surviving to find out.
From Galileo's The System of the World in Four Dialogues:
If I behold a statue of some excellent master, I say with my self: "When wilt thou know how to chizzle away the refuse of a piece of Marble, and discover so lovely a figure as lyeth hid therein?
From an interview with George RR Martin on the origins of A Song of Ice and Fire:
I don't build the world first, then write in it. I just write the story, and then put it together. Drawing a map took me, I don't know, a half-hour. You fill in a few things, then as you write more it becomes more and more alive.
There are whole books waiting to be written on the history of the metaphor of discovery used as a way of explaining the human creative process. (Who knows? Maybe these books already exist and I'm simply ignorant.) It is exceptionally common to either use the word "discovery" directly or allude to it. The person starts off with an initial seed or idea and starts from there, becoming more and more detailed and extensive in unpredictable ways. Gradually, the creator uncovers more and more of his subject - like gradually pulling a sheet away from some hidden monolith of unknown contours. The whole thing may never be truly revealed in its entirety.
I think this is why the procedural generation of things is so exciting and interesting. Whether you are rolling dice on tables to "discover" what you are going to put on the campaign map, simply filling in the blanks as the PCs interact with the setting, or rolling on random encounter tables during play and extrapolating more of the campaign world based on the results, it is like super-charging the creative discovery process. Something which is painstakingly slow and difficult in normal circumstances gets rocket boosters.