well we had an entire article explaining parts of that. You can skimp out on some areas and fool a human, but human proportions and environmental weight is hard. Then you get to motion and it's extremely hard to fool the human eye.
One end of art is spending millions of man hours to polish this effect to fool the eye. the other side simplifies the environment and focuses more on making this new environment cohesive, which relaxes our expectations. Take your favorite 90's/early 00's 3d game and compare it to Mass Effect: Andromeda to get a feel of this.
AI is promising to do the former with the costs of the latter. And so far it's maybe halfway to Andromeda in its infancy of videos.
