ID and Bethesda Softworks and both using different custom, proprietary engines. Retraining your entire studio on a new engine is extremely time consuming, especially if it's a custom engine with limited learning materials, like ID tech. There's a big cost/benefit analysis there, and frankly, if Bethesda ever did switch engines, I think they'd be more likely to go with Unreal for this reason. Current staff, and certainly new hires, are much more likely to be familiar with it.