Lurker King It is certainly true that some people can write or make art for both kinds of games; at least, I hope so! But that doesn't really prove your point. For example, take The Curse of Monkey Island. I like that game a lot -- maybe not as much as Monkey Island 2, but it's quite good. But I wouldn't assume that the writers or artists on that project would be good for an RPG -- the writing and art are a totally different style. I am maybe an odd case because Primordia was largely inspired by PS:T and Fallout; Pyke is an odd case because his games have a visual style that is extremely uncommon for adventure games but very common in RPGs. I think we might be exceptional, rather than ordinary, in regards to the transferability of our skills.
Let’s make an inventory:
Things that adventure games and cRPGs can have in common: NPCs, dialogue, exploration, items, puzzles, and story.
Things that most adventure games and cRPGs don’t have in common: reactivity, character building, skill/stat checks and combat system.
So you if you can make a decent adventure game you should at the very least be capable to implement NPCs, dialogue, exploration, items, story.
At this high level, you can draw such comparisons among many genres. For example, Tomb Raider has exploration, items, puzzles, story, and combat; Spelunky has exploration, items, puzzles, economy, and reactivity. Etc.
I think you need to look at this more closely. Then what you see is that in RPGs, items are primarily gear that affect stats, whereas in adventures, items are primarily tools that have specific scripted use. In RPGs, puzzles, to the extent they're present, tend to be relatively simple button/lever/map tricks or "riddles" or logic puzzles; but in P&C adventures, puzzles tend to be item based and more complicated. In RPGs, exploration tends to involve large areas that include considerable "dead" space; adventure games involve smaller areas where every nook and cranny is essential to find.
I want to focus on dialogue and story, though, because those are the most
important relevant to this discussion [EDIT2: poor word choice]. In adventure games, the overwhelming majority of dialogue should
not be in dialogue trees -- it should be in quips and once-off comments. This kind of writing is mostly comedic and involves a fair amount of punning, but in any event is not like RPG writing at all. Dialogue trees in adventure games should be, and are, pretty small. A single small character I wrote in TTON had more dialogue tree nodes than all the NPCs in Primordia. Regarding story, the kinds of stories in adventure games are totally different from those in RPGs. While basic writer's craft overlaps, some people are simply better at some kinds of stories. I don't think it's obvious that someone who can write a good Space Quest story could also write a good Mass Effect story.
I'm sure I can't persuade you, but to me, when I look at say, Gabriel Knight or Grim Fandango or Monkey Island, I don't see raw material that would make a good RPG, but I see beautiful adventure games based on interesting concepts.
EDIT: The other big-picture point I'd make is that all of this disregards
design, which is so important in RPGs and adventure games, and while I gather you're sort of just being glib in suggesting design isn't important, I still think the focus on art/writing is mistaken.