I think the bigger deal with Bethesda's engine is that it's built to be very easy for designers to iterate on, which is why it is also so easy for users to mod. They trade a lot of efficiency for scripting systems and level editors that let them whip up sprawling open spaces in a short amount of time, and fill them with dynamic systems like NPC routines and tracking thousands of physics-enabled props. This is probably also why their games are prone to buggy behavior.

Building all of the systems Starfield has at its disposal into Unreal would probably take years, and I'm not convinced the results would be any better.