I don't think it's quite fair to call a fandom RP unoriginal; plenty of fandom RPs are just more of an excuse to tell an original story in a world/setting that is familiar. Sure, call it an excuse to skimp on the world building or whatever but it's still at least telling a story that is new to that environment. Fandom RPs also have the benefit of attracting like-minded people, because someone who might be a bit on the reserved side sees something familiar and people who enjoy that and boom, that's how friendships form. 

Original RPs still exist, but many of them either don't get attention/off the ground or flounder shortly after launching for any number of reasons. People are drawn to the familiar, but at the same time some people who make original RP worlds or settings forget to make it attractive to the audience be it by putting a tome of information in the interest check or showing a lack of confidence in the ad or what have you. Roleplayers are often creative people, but creative doesn't perfectly equate to 'an abundance of original ideas'. Even the best original concept RPs no doubt borrowed inspiration from somewhere, because that's how creativity works. 

Originality still exists. But if people discount RPs set in fandom settings or because they don't like a genre then they might not find that it's still out there. It's like when people complain that the only movies being made are blockbusters. Sure, they're all over the place, but if you're just looking at one thing you're missing a dozen others.