Because it just can't. And it won't. It can't even reliably produce consistent shadows in a still image, so when we talk video with a moving camera, all bets are off. To create flawless movie simulations through a dynamic and rich 3D world, requires an ability of internally represent that scene with a level of accuracy which is beyond what we can hope generative models to achieve, even with the gargantuan amount of GPU-power behind ChatGPT, for example. ChatGPT, may I remind you, can't even properly simulate large-ish multiplications. I think you may need to slightly recalibrate your expectations for generative tech here.