My guess is that people are really into the whole lore and world that they have a bunch of good ideas already. Fewer people already have a whole fictional world of their own built out. And there’s a much larger audience for the existing world, that people wouldn’t be as likely to give a new world a chance.
But I think it’s mostly because people don’t care too much at the beginning whether it’ll be taken down eventually, they just have an idea they want to exist and put out there for other people to enjoy.