Because books are a different medium.
That doesn't explain why you think* it's okay to present a person's inner thoughts on paper, but not on film.
*not that there's anything wrong with thinking that, but your wording implies that you think it's an objective truth.
because books can be written in third person or "God" mode
And films are (usually) shot in third person or "God" mode, so what's the difference there?
In case I wasn't clear (I hadn't realised you haven't read the books before) I was specifically talking about the Dune books when I mentioned literary inner thoughts. They're in there, just like they are in the film, and they're integral to understanding the characters. There's no way you could convey their meaning with a bit of "smell the fart" acting.
but expressions and actions are only described, not seen.
I don't see how that has any bearing on the portroyal of inner thoughts. In books, they're written down. In films, they're spoken. I don't see why one should be inherently better than the other.