What I don't generate randomly or spin off a secret, I steal. Shamelessly.
Partially because it helps maximize content volume while minimizing work. (Though I tend to give everything my own spin, and at the least have to translate it into game-meaningful content, which is the most work intensive part. So it doesn't minimize effort that much.)
Partially because I like the easter egg aspect it lends to the game. This comes up a lot with character names--names are the most easily Google-able. This would probably drive some people crazy, but I think it's funny when the players finally realize exactly where I got the name of that ancient artifact sword.
Mostly, though, I just like using things I think are cool in my game. I prefer roleplaying that focuses on emulation, rather than innovation. It plays more to the form's strengths. Using familiar material helps keep the inevitable misinterpretation that comes with a group of people trying to figure out what's happening through verbal interaction down to a manageable level. And one of the big advantages that roleplaying has over less active forms of entertainment is that you are actually involved, on some level, with what's going on; using material that you already think is cool takes advantage of that.
And, on an even more theoretical level, I am not much for pure and total originality in creative processes. There are hard limits to just how original it's possible to be, and even harder limits on how original it's desirable to be. If I have to choose between innovation in premise and competence in execution, I almost always choose competence.