Why are surgeons not able to tell if a patient has regained consciousness during a surgery by simply observing them for muscle movement or listening for speech sounds?

The surgeons will not be able to tell if the patient has regained consciousness with the following actions given above because it is not enough to prove that the patient is conscious and well after the surgery. The only way to prove if the patient that has regained consciousness is when he or she has awakened and finally was able to talk or do things normally before he or she has even gone through surgery.