So, as others pointed out acetaminophen is actually fairly dangerous as drugs go. However, let's pick on something like ibuprofen instead which is definitely safer.
Today even ibuprofen would have trouble making it as a non-prescription drug.
Pain-killers in general have the deck stacked against them. For something like a heart medication to get on the market you basically have to show that it saves more lives than it takes. So, if it prevents 10k more heart attacks per year than any other drug on the market, and it kills 10 people per year due to liver toxicity, then it isn't hard to get it approved.
Painkillers don't benefit from this kind of calculus. If it kills 3 people a year, you can't point to a single life that they save to balance it out. So, our regulatory system tends to keep painkillers off the market. It is hard to balance lives cut short vs long lives lived in agony.
Diet medications have a lot of problems with suicide and tend to be kept off the market for the same reason. (Which makes me really wonder about the interaction between diet, obesity, and depression - eating is a basic instinct and we already know that people eat when they're upset - the need to eat is in many ways driven by emotion.)