I think perhaps that only children tend to spend more time in the company of adults and are exposed to adult dialog, conversation, literature and culture at an earlier age. Just from my experience (as an only child) that was the case. From an early age my mother was taking me to museums, operas, and historical places on my own and I spent probably as much time with her adult friends and coworkers as I did with kids my own age in school. I didn't have the "distraction" of a sibling and while I certainly played with kids toys and watched some kids' tv, those weren't the only things I was exposed to.
Only children, in general, also spend a lot more time by themselves and come to value/need that "alone time/space" to think, dream, explore individual activities. And when you read, explore and question things on your own, you're likely to develop a more expansive worldview. To be less conservative and close-minded.