>>28508386PFAS from (among other things but most famously) nonstick coating manufacturing wasn't properly studied before mass production, and once they found out it bioaccumulates and doesn't break down for years (the nickname "forever chemicals" comes from their stability in the environment) they just kept producing it anyway. Since it contaminates water runoff and doesn't get properly filtered, it just got into the water table everywhere and circulated around the whole planet over the course of decades. It's one of those things like microplastics where we just hope the long term effects are minor (we already know they aren't entirely but there's nothing to be done) because fixing it will require technology we don't have yet, but there isn't any legal system that was made to handle a crime on the level of "poisoned all of humankind" so instead of trying the governments of the world just never bothered pursue legal action against DuPont.
The *good* news is that it looks like some bacteria are already capable of processing PFAS and can get along just fine in the human gut, so we might not have it as a persistent issue. The downside is a lot of modern convenience goods still require PFAS-producing stuff and it'd be politically impossible to ban them all.