Dungeons are a great place to get some game on, and they serve some purpose in the greater world. They can be abandoned mines, or whatever. The world is OLD, and it's much easier for some tribe (of a race you've never heard of) to move into a dungeon built a thousand years ago by some ancient and long dead race they'd never heard of, and then modify it a bit. When they died out in turn, the cycle repeats itself. You get some weird ass dungeons lying around with a history like that.
If you think about it, dungeons are much more likely to be lived in and really, really old than cities are. Cities are subject to weather. Buildings burn down. None of that happens to dungeons.
My dungeons aren't completely random though. I probably wouldn't have the proverbial Ki-Rin and Balor neighbors. The dungeons make some sense for their current occupants, even if the architecture doesn't and some rooms go unused. That seems pretty standard though. I have some old 1e adventures, and that's what they're like. The monsters that are in the area are aware of each other and respond rationally. I, personally, haven't run into any truly random dungeons. Maybe I'm playing the wrong ones?
No, the dungeons are not lost. If DM's talk about campaigns now it's because the dungeons are just understood to be a part of the larger campaign. When I mention the capital city of the Zor Empire, it's understood there's a labyrinth of secret passages, sewers, and the remains of the city that came before it, layered several stories deep. If I'm telling some other DM "Oh, and in these mountains the goblins have completely run over the orcs", do I really have to point out that the Orcs riddled the mountains with tunnels, and that only half of them have been cleared and occupied by the goblins? Isn't that just assumed?
(Obviously not everyone thinks that, I know. Well, I guess your goblins are just more efficient about clearing out tunnels than my goblins. My goblins are lazy, and if the bugbear doesn't bother them, they ain't gonna do nothin' 'bout it.)
Also, as for plot, I've never, since my 1e days, gone into a dungeon without some reason. It was pretty flimsy in the beginning, and roleplaying was minimal, but there was always a princess to save or a monster to hunt down that had eaten a couple villagers. Are there people out there that on a regular basis just walked around rolling dice with no higher purpose but to see if you survive a particular encounter? Once maybe, but I bet most of them don't play D&D anymore. They just play Tekken or Diablo 100% of the time. It's easier.
Last time I checked, this game was still called D&D.