There is a lot of difference between working alone in your own world (when you are free to inject your own tastes, opinions and biases), and building on an existing intellectual property owned by a third party.
GRR Martin writes the stories of the Seven Kingdoms by himself, of course, but he is both the creator, the owner and the writer of the saga. He doesn't have to answer to anybody. By contrast, Runeterra's IP is owned by Riot Games, and they decide what flies and what doesn't. In addition, if you put all the decision power in a few people, what happens if they leave the company?
Even more, one of your examples fails to prove your point: George Lucas surely came up with the basic idea for Star Wars, but the original script was rubbish. A huge amount of Star Wars' awesomeness came from input from other people.