I worry sometimes that I've developed less of an attention span over the years of playing casual games.
I know what you mean.
I think it's a form of burnout -- less patience with the parts of games you don't enjoy. The casual adventures allow you to skip past conversations and "story elements" and get on with the puzzling. And then the "type" of puzzles you encounter in casual games are often more enjoyable than the limited types you find in most current adventure games. Some of us enjoy "door puzzles" and "abstract puzzles" and "mechanical puzzles" and don't much care if they have anything to do with the "story" or not. And if the "story" is some hokey variation on a theme we've seen 16,000 times before, we'd rather have as little to do with it as possible.
The first adventure games I played were 1st person puzzle-oriented games where any "character interaction" was limited to a handful of cut scenes. So the first time I encountered a game with dialogue (in my case, it was Titanic: Adventure Out of Time), it was a novelty to be able to interact with other characters and have conversations with them. But by 2014, conversations have long since ceased to be a novelty, and in some cases they comprise the bulk of the game. At some point the experience changes from "me playing the game" to "the game playing me" -- telling me where to go and what I can and can't do rather than me exploring and figuring out what to do on my own. At that point, I want to leave and go somewhere else.