My husband had a scope from hell too. He flexed so hard in pain that he broke the blood pressure cuff. But he never said stop because he wanted it over with.
Here is my take on why they don't stop. With conscious sedation, even though you don't remember anything and think you are out, you are actually awake. You just don't remember. So, I bet people say stop all the time, even when the meds are working, and just don't remember. So when you really feel it and REALLY say stop, they don't know the difference.
Either way, SCARY.
See, that is just cruel to me. We'll put you through pain and you'll be totally aware at the time. But later you won't remember a thing. Just seems so inhumane to me. They wonder why so many people don't have these tests when they really need them. When you first have to go through a terrible prep, then that! I just say, knock me out.