|Oh, I see.|
It seems that for a lot of newer players a "campaign" is basically a process whereby a group plays through a pre-set list of modules and then calls it a day.
This is like saying that to really enjoy Art you must visit a pre-set list of the world's great museums - it confuses cause and effect. An art lover will probably visit as many of the world's great museums as they can, but they usually do it because they are art lovers already.
Similarly, many old duffers will tell you tales of how much fun they had playing through G1-3, D1-3, and (sometimes) Q1 (more rarely, A1-4). But there's no way in which that list constitutes a campaign in the sense used in the DMG. At most, it's part of a campaign. A small part at that, in my experience.
While our old group did indeed play through these modules (except the A-series which had real problems), they constituted only an incidental part of the campaign. The vast majority of the campaign revolved around the characters' actions in a sandboxed Greyhawk setting where events evolved out of the character's previous actions. Modules were hauled out only when things were quiet.
And "campaign" originally meant a little bit more than the idea that a party's adventures were linked; it meant a setting where more than one party of characters (and players) was active, allowing for one to affect the other. Personally, I only ever managed this with my CSIO campaign and it was a lot of fun seeing the players try to make sense of the other group's trail of destruction (and occasional shrine building).
But the idea that a simple list of maybe 11 modules might constitute a campaign has puzzled me for a while and I only recently, while scanning the bizarre world of rpg.stackexchange, realised what the root of this attitude might be. It seems that a lot of players are approaching the game (or at least the so-called fifth edition of the game) from the perspective of it being like a board game with an end goal and, therefore, a pre-defined win condition. The condition in this case being an arbitrary character level.
Under this model, the point of the game is to find the route to that level which is most efficient, i.e., quickest, and to traverse that route with the least fuss or, indeed, effort.
Realising this suddenly made sense of a lot of comments I had read over the years about the various copies of the game, not only the non-TSR versions either - this dates back to late 2e games I remember people talking about.
Basically, these players have mis-understood the whole concept of the role-playing game, and D&D in particular. They have swapped the side effect (high-level) for the goal (playing fantasy characters having adventures in a fantasy world).
The result is a massive drop in potential fun. Having "wrapped up" one of these severely curtailed "campaigns" many groups, it seems to me, move on to the next game system. Whereas many Old School games continued for years and decades, these shadow campaigns are over and done with sometimes in months, and almost always in a couple of years of not particularly intense play. And although they're presumably fun for the players while they last, they're very much at the "wham-bang, thank you, ma'am" end of the satisfaction range.
Well, this wonderful observation may not be news to anyone else, but I certainly feel like I at least understand the mistake that's being made a little better and while phrases like "we're playing a campaign to 20th level" have always irked me and probably always will, I can see where they're coming from now.
So, in summary: high-level isn't important, the story you tell while getting there is.