Hobo said:
This is the old "talking about a game vs. playing it" argument. So for example, there are a lot of people talking about how popular a game is, but I often find they can't find other folks to play with, or a GM to host. You can, in theory, play with as few as one player but as many as will fit in the room with the GM. You cannot play without a GM.
So while GMs are in the minority, they are a very important lynchpin of a gaming group. More importantly, they are often a distinct personality type, usually but not always the organizer of the gaming event. They are hub people, the people that virtual marketers drool over because they influence others. In the role-playing instance, this role is codified -- they literally determine life and death in a game.
Sourcebooks can be used by anybody. If I play a character from Coruscant, I probably want the Coruscant book. If my GM is hosting a game in Coruscant, he probably wants the book too. So we both need it.
As a player, I don't need an adventure set in Coruscant, only the GM does. Simple math, right? If we assume four players per 1 GM, which would a company rather sell, five copies of a setting book or one copy of an adventure per five people?
BUT.
Putting an adventure in Coruscant may well trigger the interest in Coruscant. If that adventure spawns other adventures on that planet, my GM may then feel inclined to buy the sourcebook -- the other players may feel the same. If there are maps of ships from Coruscant in other supplements, I may want to buy those too. Music from Coruscant? Video from Coruscant? Miniatures from Coruscant? I know have an excuse to buy all that stuff.
In the short term, sure, one adventure only sells to one person. In reality, it selling an EXPERIENCE, and that experience is an intricate web of products that can make more for the company in the long-term.
You can see this happening with D&D minis, wherein DMs become mad completists, determined to get the exact quantities of monsters listed in a module. I know I did this -- I went out and bought monsters for everything the PCs might encounter, and the adventure acted as a shopping list of sorts. Adventure in a haunted house? I buy the haunted house tiles from Paizo. Attacked by two otyughs? I scour the D&D mini singles to buy it. I shudder to look at what it REALLY cost me when I consider how much I bought to complement the adventure, but it could easily be over a hundred bucks.
Adventures tie all the other gaming media together.
Hobo said:
GURPS. Exalted. Vampire. Star Wars.
All of them (in fact, almost every game line except for D&D and CoC) have very little to no adventure support.
GURPS: Pyramid does an excellent job of supporting GURPS with scenarios. I've used quite a few of them for other games.
Star Wars had great heaping piles of adventures when it was first produced for the d6 system. I'm not familiar enough with the current d20 system iteration to know how much support it has received. I believe there was a Star Wars magazine for RPG support as well, but I think that's since been discontinued.
I'm not familiar enough with the Exalted line, but I think you're right, a popular game line without support in that case. Vampire is questionable, because there are adventures published for it, but they tend to be published as narrative arcs rather than specific adventures. There's also the generic Storyteller system which mucks with that a bit, because I think there are scenarios that could be used for Vampire but aren't specifically for Vampire games.
CoC is unusual in having entire campaigns as its most prominent source material, and D&D is unusual in having a high degree of modules/adventures published for it.
You seem to be trying to say that publishing adventures is the rule rather than the exception, but in my experience, the exact opposite is true.
I dunno, I think two highly popular game lines, with one of them being the biggest RPG line, are hardly aberrations.