It's intellectual property. It can't be used without the creator's consent.
Intellectual property (IP) is a number of distinct types of legal monopolies over creations of the mind, both artistic and commercial, and the corresponding fields of law. Under intellectual property law, owners are granted certain exclusive rights to a variety of intangible assets, such as musical, literary, and artistic works; ideas, discoveries and inventions; and words, phrases, symbols, and designs. Common types of intellectual property include copyrights, trademarks, patents, industrial design rights and trade secrets in some jurisdictions.
Well they better copyright this then. ;)
The thing is, most game studios already have a game concept staff, and to be honest, it is one of the easiest things to do when making a game. Besides, most of the time, the people in charge want to make a game based on what THEY think would be awesome, not what other people do. ;) Besides, a lot of this ambitious stuff would be cool, but the reason other MMOs do not have it is because it just does not work on a large scale.
"Don't stop, believin! Hold on to that feeeEEEeeeEEeelin!"