I think part of it is the technical mastery on display. It’s like The Matrix at times.
NPCs have houses, jobs, they cook, clean, eat.
If you knock somebody out and another NPC sees you, they grass you up to law enforcement later.
NPCs walk and ride between towns to trade, they clean clothes in rivers, they fish etc etc.
There’s actual forest sized forests, villages, towns and a city which feels 10 times bigger than Whiterun in Skyrim.
Also, the game hates you and wants you to die.