That's not how it works, first of all IdTech is not a general engine, it's only made to build specific games: fps.

Next you need to make a product out of that meaning a very large investment so that the engine is actually usable outside of ID, people that don't know what an engine is think of the rendering part but it's actually very minor compare to pipelines ect ... which we know nothing about, it might not even be good or painful to use.

Unreal is widely used because it can create any kind of game and the pipeline, tooling are good and well integrated.