This is inspired by the conversation Lynda had with the Doctor in "Bad Wolf", about how the Earth just "froze". Love to betterwithtwo at T&C for the transcript!
People say it wasn't always like this.
Stories. Stories of the old Earth. The great Earth. The human world. Interwoven with local legend and rumours of the old. Generations taking part in a game of fanciful Chinese Whispers, twisting and distorting the facts until no-one knows. Until no-one can be sure of what the world should be like. Anyone alive back then is gone. Leaving behind the stories and the hearsay. The myths. The legends. Passed down through the ears of children, and their children. Fable legacies.
They add their own ideals to the fantasy. Creating their own vision of paradise. And it matters that what they dream of is only that. A dream. They all know, deep down, that it's pointless wishing. This is the way the Earth is now.
And this is the way it will stay.