I'd like to propose that original ideas aren't as uncommon as they seem, but plenty of them tend to borrow tropes and cliches that tend to shove them under the radar. They appear less attractive because they have ideas used time immemorial or come with massive lore dumps involving the world. I feel most players want to feel like the belong to the world instead of acting as a guest, so it's a major turnoff to throw yourself into a world that's already been pre-established. Especially one that you had no impact in making.
I think you're touching on something very true there, Hyde. The old saying goes "nothing is original" and it rings true today. Nearly all of our writing deals with common tropes and themes seen in human culture for thousands of years, either playing the theme straight, subverting it or trying to paint it a different shade, but we're still playing with these common themes or tropes. There's nothing wrong with using these things, as they are the building blocks of our culture, beliefs and entertainment. But the classic heroes journey has been a million times and will be done a billion times more before the heat death of the universe.
The other aspect I find interesting is your viewpoint on "being a guest in the world" vs "being a part of the world". I cut my teeth nearly two decades ago with the latter; spending every Friday night with friends in a chatroom pitching out ideas and building worlds and stories together; which can be fun. Nowadays I spend more time building up the background and world in general before presenting an idea to my groups, mainly because many times people can feel overwhelmed when presented with too much choice in how to design a character. Eg, "It's a fantasy world but how does the magic work? Am I a Gandalf style wizard with natural power, or is this more Final Fantasy 7 where I have to slot the ability to use magic into a weapon". I think at the very least having a set of rules and reasons for the world is good before dropping ideas to total strangers.
I do agree with you though. I think giving players the freedom to create their own part of the world is important. The best kind of original world RPs act as a balance between the extreme worldbuilding and extreme freedom that have been discussed. That's why I roleplay, after all. Different players from different walks of like have time and time again surprised me by how they can look at a theme or situation with a completely different eye than myself, and I find that extremely invaluable as a writer.