It's more baked into their game design. Everyone knows their games are huge, but they also populate their worlds with fully interactive items everywhere that never disappear and each item can potentially interact with every other item in some way. On top of nearly every NPC having its own AI schedule, navigation route and all these control meshes are interacting with each othe constantly. From a coding perspective, thats an absolutely nightmare.
We have people writing articles about how difficult it is to get a
door to work in a game:
https://www.theverge.com/22328169/game-development-doors-design-difficult
Then think of NPC acting independently of each other in a giant ass game where every item is permanent and can be interacted with. The logistics of their game design are insane and it's impressive that it actually works at all. Bethesda aren't the only devs to make open world RPGs, but nobody else does an open world the same way Bethesda does. It's not necessarily better or worse. It's a double edged sword, but for many people the trade off is worth it. There's not a lot of games that get as much track as Bethesdas games as far as longevity goes.
And there's yet to be a Witcher grandma.
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