Michelle Nephew and Robin Laws, both game designers, have thought deeply and written extensively about the origins of the collaborative nature of RPGs. Between the two of them, we get a good account of why D&D appeared where and when it did, though neither Nephew nor Laws quite agrees that RPGs are uniquely modern.
Laws emphasizes the importance of where all those early wargame innovators come from: Iowa, Minnesota, Wisconsin. For Laws, a game of cooperative effort, like Dungeons & Dragons, "had to be an expression of the Midwest." The consensus gameplay of RPGs reflects the "friendliness and laid-back sense of togetherness" of the place where they were invented, a place with a communitarian history. Surviving together in a dungeon parallels surviving together on the American frontier. This geographical influence has continued to the present, so that with the development of 4th Edition D&D, we're seeing, says Laws, "a battle between corporate interests and the traditional interests of the gaming community." The conflict Laws is talking about arises from the fact that, in the RPG industry, gameplay serves as a business model.
Roll for Licensing
Wizards of the Coast, the Hasbro subsidiary that makes Dungeons & Dragons, has taken great pains to develop its Game System License, a legal framework allowing third-party publishers to produce material compatible with D&D. With this document, Hasbro carefully dispenses access to the intellectual property of Dungeons & Dragons. Such a maneuver would be insane in any other industry, but it makes sense for roleplaying games, because they inculcate habits of collaboration and appropriation in players. Hasbro must make its money, but players have to feel that D&D is still a product of their community.
At the other end of the business scale, independent game designers come to informal arrangements with their community. They frequently release their work under the creative commons license, which can allow borrowing of proprietary material. Some online communities of designers, such as The Forge, appeal to "mutualism" in game development, which can mean an exchange of ideas, criticism or even labor: I do artwork for your game; you show me how to put mine on a PDF. Many indie games are very close to being products of an anarchist collective.
Whether corporate or collective, the RPG enterprise comes back to just the sort of group dynamic that you find in an individual game of D&D. The conflict over how open and consensual to make things is part of the creative process.