Why is it called Game Engine, not rendering engine?
because bethesda licensed only part of it, plus game engines arnt one stop shops for all your game developing needs alot of other stuff including a large amount of hard coding and modifications go into it, i dont know a single company other than EPIC who takes an engine out of the box and only uses the tools inherent in it. Devs code new tools, and tech as well as license bits and pieces from all over the place.
Engines are collections of tools thats all, so the tools from Gamebyro that where used include only the rendering software