Your personal experience of having never encountered a traditional dungeon, to the extent that you seem even to doubt that Gygax intended it to be normative, is suggestive.
Here's a question, since you assume it was to be the norm of play that there would be a superdungeon as the centerpiece of a campaign: why did TSR never publish any supplement of this nature in the 1e era?
Because the underworld was not the ONLY element of the campaign! Is this really so difficult to understand?
And the problem with the megadungeon assumption you seem to think is in the books is that the material focusing on that is limited at best, and for the most part confined to appendixes. Whereas the material suggesting otherwise is much more extensive and part of the main body of text.