Sure, unscripted moments can be an asset, but they aren’t the thing that makes a move “realistic and immersive”. Plenty of great directors are strict and won’t do anything that isn’t on the page, even if they otherwise have a very naturalistic style.

Plus, there’s a difference between allowing for an actor improvising a reaction or a line and… you know, forgetting to erase a technician from a green screen shot for a few frames. I don’t know that counts are an unscripted moment.