For an established studio, switching engines is a big ask. It means retraining the whole team on a completely different set of tools.
BioWare used to develop UE-based games. Mass Effect 1, 2, and 3 were Unreal Engine 3-based. Then, EA forced them to switch to Frostbite (the Battlefield engine), because they were greedy fuckers and didn't want to pay royalties for UE. Frostbite, at the time, had basically no documentation and didn't even have a savegame system. BioWare had to code one into it for Dragon Age: Inquisition, and they still shit the bed with Mass Effect: Andromeda and Anthem.
Bethesda's Gamebryo fork has its advantages. Namely, it allows retarded-monkey-level developers to snap together prefab pieces very easily and slap them onto some heightmapped terrain and call it a day. The phoneme-based dialogue system means that there's no need to mocap every little interaction. It serves their niche well, however, the fundamental framework of Gamebryo is very long in the tooth.