Why would they leave out McMurphy ripping open Ratched's shirt, and Ratched being unable to talk?

Frankly I would not want to see that. I also would not want to see the abuse done to the patients in the book.

Seeing something like that in live action might elicit sympathy for Ratchet or potentially be seen in a comedic light (similar scenes were being done in sex comedies not too long after this movie). It's something that works in prose but not on film.