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.

I mean, to be fair, Starfield doesn't do it well either. In the 15 hours I played, especially toward the latter end, I ran into plenty of texture pop-in, bad culling, bodies without heads and arms, heads and arms without bodies, bad shading patches, t-posing, stutter, lots of other goofy shit. And granted, my rig's not the best but I'm playing on medium with a 9600K, 3070, 32GB RAM, and the game's installed on a Samsung 870 SATA.

A loading screen lets you load different areas of the game discreetly and make the game performative. This is especially important as Starfield is a single player game, it’s not hosted on a server or anything so it can’t distribute resource load that way, its all happening client side on the player’s system. They would have to simulate the entire world on their PC alone or develop a way to stream the content out dynamically and seamlessly.

That's not how any of it works.

We have had level streaming in Unreal for like a decade. Sure it's more complex to do things this way, but in general the way it works is that when you approach some area (are some distance from a planet or part of a planet) the next chunk of the world loads in, together with any NPCs and logic and everything else - it's basically a self contained map, just seamlessly integrated with other maps. There is no meaningful performance hit if done correctly. You certainly don't simulate everything all the time.

Additionally, all the other games mentioned (NMS, Elite, Star Citizen) also have basically all of the processing on the client side. The servers don't help the clients in any way; they only store primitive states for gameplay purposes, but all the simulation and whatnot is done on the client. And they still manage to be better optimized.