The problem is that N-acetylcysteine tastes and smells awful (like farts/rotten eggs) and often causes nausea and vomiting as a side effect.
The vast majority of people use acetaminophen in a safe way, and acetaminophen doesn't really have many side effects by itself, so you'd make life more unpleasant for a large number of people in order to prevent a tiny number of acute poisonings.
Probably if this were implemented, most acetaminophen users would switch to e.g. ibuprofen which is less acutely toxic in overdose but has much more chronic toxicity (to the stomach and the kidneys) when used over a long period of time, even at a normal dose. I'd wager this change might even be a net negative on the whole.
The vast majority of people use acetaminophen in a safe way, and acetaminophen doesn't really have many side effects by itself, so you'd make life more unpleasant for a large number of people in order to prevent a tiny number of acute poisonings.
Probably if this were implemented, most acetaminophen users would switch to e.g. ibuprofen which is less acutely toxic in overdose but has much more chronic toxicity (to the stomach and the kidneys) when used over a long period of time, even at a normal dose. I'd wager this change might even be a net negative on the whole.