idTech is cutting edge tech. Their team is one of the best in the industry, second to none, competing with Epic, Insomniac and Naughty Dog. But it is not engine for general use, not now. It does one thing(FPS), and does it extremely well. But it lacks tools that you would need to create games of another genres. Things like advanced animation tools, dialogue systems, quest systems, ways to handle vast open worlds, etc.

idTech would be great for Halo and Call of Duty. But it isn't great for The Elder Scrolls, Starcraft, Gears, and many different games Microsoft Studios are working on. EA already tried to make every studio to use Frostbite for every single game and ended up with disasters like Dragon Age Inquisition and Mass Effect Andromeda.