Skyrim, FO4, and almost any close-to-mainstream games usually cannot legally claim they run different engines. Because usually they all use the newest Unreal Engine.
You are mixing up and generalizing two different things here.
Bethesda is still using the Gamebryo engine, which ironically was used for Civilization 4 as well IIRC, they just rebranded it "Creation" to imply it would be something new, because they are too cheap to aquire spmething new.
The Unreal engine itself is mostly used as an affordable engine by smaller companies without access to inhouse solutions, but it also serves as a foundation to create something new, like EA's Frostbite engine. While it is still "based" on it, it does deserve it's own name. It just becomes a problem when marketing slaps a v2 on it just because someone changed a shader.
Pure Unreal engines are mostly used for consoletard games these days, so they can quickly pump out their shovelware on a proven plattform
But the big "mainstream" players like EA, Rockstar, Ubisoft or CDPR have all built their own engines.
Probably because the Unreal engine fees are based on sales.
