Seems id's shiny new Tech 5 engine wasn't built for Rage, even though it'll be the first game to feature it. id's Todd Hollenshead:
We didn't design the game for Rage and then John Carmack worked on the technology to make it work; John was working on some technology and we thought we had one direction we wanted to go in in respect to a different game. But as technology evolved it basically got about a year into it and effectively we sad 'we're going to throw all that away and start a new project.
Shame. As much as it's probable the game was a dark, grimy sci-fi shooter, I'd like to think it was a bright, cheery, first-person adventure. With puppies and rainbows. New engine wasn't for Rage, Id reveals [CVG]