From my understanding game developers used mostly proprietary, in-house graphics engines during the sixth console generation (Xbox, PS2, GC, DC) on a game to game basis. Why didn't standard engines like Unreal (for consoles), Unity etc. emerge earlier? Wasn't it very inefficient to develop a new engine just for one (or probably a few) games?
Bonus question: Are there any good ressources on graphics development during that time? I'm especially interested in articles about the effort needed to develop an engine for any of the mentioned consoles.