*** Dehorning adults is both, as you said, gruesome and more physically traumatic and bloody to the animal.***
And yet, when you release the dehorned animal it will go into the pasture and start eating. Same for branding. Let them up and they will start eating. They do not set around and bawl about their wounds.
Studying emotion in psych and in a video they mentioned the same thing about other animals after nearly being killed, even after a lion attack. would be a good study to find out why but the reason could very be that we wonder why.