From featured comment in Ars Technica
The biggest impediment to large open-world games is the cost of asset development. What you ultimately want to be able to do is take a model of NYC and use that as your starting point for either Spiderman or The Division and not have to recreate those assets over and over.
That's really what Epic is focused on here, and surely it derives from the work they're doing with movie studios. These are all problems that they are encountering using Unreal for filming. But now Marvel could build a Mandalorian game around the digital assets they created for the show. Everything is already in-engine. There's still a lot of work to do around gameplay, actors, scripting, dialogue, etc. but having so much of the world assets already done seriously eases the cost and length of development