It occurs to me that books and movies are basically an externalization of the roleplay all human beings naturally engage in as children.
In other words, we never really stop roleplaying as we grow into adults. We just rely increasingly on the imaginations of others... entertaining their adventures, rather than conjuring our own.
Instead of donning paper helmets and swinging wooden swords, shooting paintball guns and lobbing water balloons, we watch action movies, violent and competitive sports, and military documentaries. The girl who plays with dolls and tea sets might grow up to watch melodramas and "play" hostess at gatherings. It's still playing pretend, but why come up with your own fantasies when someone else can provide better ones?
If this is true, then it makes sense that video games would be tagged as childish, because the medium is a step back toward the self-directed roleplay of childhood.
It also suggests that role-playing games are not just for nerds. They're for everyone. If RPGs have trouble matching the sales of shooters, perhaps the problem isn't the genre itself. Perhaps the problem is a mistake in how RPGS are commonly realized.
It's not that RPGs are intellectual. Football has an abundance of rules and more strategy than chess. But football is also fundamentally about action. And there, I think, is the key. It's the poor presence and implementation of action that holds most RPGs back from broad appeal.
On the one hand, you have RPGs like Final Fantasy and Mass Effect which emphasize dialog and cutscenes over other elements. On the other hand, you have MMOs and D&D-derived stat-based combat systems in which the rules dictate more than guide.
I'm happy to see a lot of blending between genres these days; particularly between shooters and RPGs. I think developers will find that most people, and not just nerds or hardcore gamers, respond well to RPG elements when those elements don't slow things down to a crawl and allow players the freedom to direct their own experiences.