Because Doom 3 was released 7 years ago and the engine was probably in major development what, 8-9 years ago? They've since moved on and their most recent game

Because Doom 3 was released 7 years ago and the engine was probably in major development what, 8-9 years ago? They've since moved on and their most recent game, RAGE, uses the fifth version of their engine, not the fourth. They're basically open sourcing the engine that they're no longer hyping/selling, while they continue to use their modern engine for business. How can they? I mean, I guess other companies just lets their engines die instead of open sourcing it.