Crap, this again?

Everything that needs to be said was covered by the others. But basically, it boils down to existing fandoms =/= devoid of creativity. Many of the "original" RPs use concepts that are frequently used, and as Kaga said, if you make up too much stuff for an original game, it may make sense in [I]your[/I] head, but keep in mind that nobody else was working for however long to bring that world to life. They're coming into it, and trying their best to keep things straight without being contradictory or breaking the original game's lore. It's a huge amount of work that people constantly have to second guess themselves if they aren't sure of something, which is why a lot of RPs, fandoms or original, use things people are familiar with. It's not because the GMs are hacks with no creative spark, it's because they rather focus on building a story that players immediately grasp instead of being constantly interrupted by the fact that several of the players may be forced to take educated guesses about stuff a GM made up and hope they didn't screw it up, which grinds things to a halt, forces edits and can often lead to rather forceful disagreements and frustration. 

Thus why people like me like to use established settings from games I rather enjoy the setting and lore of; it doesn't mean I want to tell the same story as the game that inspired the roleplay, it means I enjoyed something enough that I want to tell a story in that setting because the universe is big and interesting enough that I am confident of crafting an original story in those confines and that all the players coming into it are on the same page as I am. It's hard for players to have arguments about a setting if the rules are already well established. Plus, I tend not to use anything from the games that inspired the roleplay; nobody wants to play a plot they've already participated in before in another medium. This is a big reason why my Mass Effect RP takes place a decade before the first game; absolutely nothing from the game series plot and characters will make an appearance. Likewise, my Elder Scrolls game is a plot that has nothing to do with the plots of the existing games. But what do I know? The former's been running for a year, and the latter is about 3/4 of a year. Having a game last that long is something very, very few games can boast. And you don't have a game run that long if the people involved are uncreative, uninspired people who parrot everything they see.