Imagine a person who only consumes Batman-related media. That is, they only watch movies and TV shows that have Batman in them, only read books that are novelisations of Batman media, only play licensed Batman video games, and so forth. This is not so absurd an idea; Batman-related media is sufficiently popular, varied and widespread that restricting one's media consumption in this way is completely feasible. However, I trust we can agree that if you actually do this, you will be left with very strange ideas about what popular media looks like.
The next step in this analogy is undestanding that if the only tabletop RPG you're acquainted with is Dungeons & Dragons, you have the same grasp of the tabletop roleplaying hobby as our hypothetical Batman Guy has of popular media.