I assume you mean "why can't glutathione, NAC .. be added"
There is one problem. The FDA prohibits combining existing drugs and
dietary supplements unless a New Drug Application is filed, tens of
millions of dollars of clinical studies are performed, and the FDA
agrees to allow the combination to be sold. The whole process can cost
upwards of $100 million and take a decade to complete.
It's sad that would would be very likely an effective solution for the deaths and liver damage is not available or made more widely known.
Only about 10% of acetaminophen is metabolised through the glutathione conjugation pathway via NAQBI (the other 90% is direct sulfation and glucuronidation of acetaminophen). Many people take NAC supplements regularly (I used to). While high does can have some side effects, a typical supplement level dose of a few hundred milligrams daily are well tolerated.