I tend to have a hard time describing my campaigns to non-participants. You'll note, for instance, that my "recaps" here tend to focus on a few key issues, often mostly out-of-game or otherwise technical in nature, with very little discussion of any events outside of the specific problem I'm having. If I did more than that, it would quickly turn into pages of material.
The Traveller game has already gotten to the point where it would take several paragraphs just to fully discuss their current mission, and explain why everyone's going along with it, how they got it in the first place, and where they're going from there. A complete explanation of all the different goals they have bubbling on various burners would take weeks.
It's ridiculous. But it's not unusual, for one of my campaigns or for roleplaying campaigns generally. This one doesn't even involve all that much history; it wasn't unusual in Is This Fair for a complete update of the campaigns current status to require repeated references to events that happened in two distinct time periods, one thousands of years prior to the start of the campaign, and the other around four hundred years previous.
I'm sure that there are some campaigns which don't develop such labyrinthine complexity, but it strikes me as a general tendency of such things, if only because there's half a dozen people all contributing their input to the thing that will someday resemble a plot. The basic challenge of integrating four or five separate backstories and character motivations into a coherent whole, which a campaign does by default (though a good referee will guide it towards a stable state that doesn't shortchange any one character or group of characters in favor of another) accounts for a great deal of complexity all on its own.
But a lot of it comes directly from the game master's side of the screen. This has been particularly true in my current campaign, but it's true in a general way of most roleplaying campaigns: I generate a lot more material than I will ever reasonably use, most of it in the form of offhand references, minor NPCs, and hooks the players don't bite. This is just a normal part of negotiating the divide between player and game master; I don't know exactly what they'll be interested in, so I make more than I need, and they'll be more interested in some things than others, so they'll notice less than I make.
Sooner or later, though, some of that material that doesn't immediately get used will come up again. Either the players will remember what I'd intended as a one-off incident and attach some significance to it, or I'll reason that something that happened a couple of sessions ago has a neat connection to what they're doing now. There's a continual process, negotiated between me and the players, of generating material, reacting to it, creating a story to explain it, and then generating more material based on that story.