I'm talking about the Creation/Gamebryo specific sets of mechanics like NPC schedules and the radiant AI or quest systems, those specific things that needed to be created that aren't inherent in the engine. Not that Starfield is really the best show for those anymore, not by a long shot, it's more of just an example of a limitation of OP's idea of all devs using one engine.

Developers could all use Unreal, but if someone wanted to make Oblivion on Unreal they'd have to program and create those systems and mechanics because they don't just "come with the engine". If they made those, and all devs use Unreal, should they be folded into Unreal for future devs to use? Should Unreal program those mechanics or something similar for future devs to use? At what point does it become too complex to bolt on certain systems to an existing engine instead of make one explicitly for it, depending on the type of game?

I don't have a great example for a game so novel in its execution that it would be truly limited by Unreal, because that engine is absolutely powerful, it's more thinking about what would happen in a world with a single engine monopoly. Some studios would end up with their own proprietary offshoot modded engines like all the engines that spawned out of modified Quake engines back in the day, for instance, goldsrc.