In the US specifically, it is class actions that make it expensive. If someone accidentally or deliberately puts lead in the flour supply, then that can probably be traced to tens of millions of cakes given to tens of millions of people.

And a good prosecution would be able to prove that while a small amount of lead didn't cause noticeable symptoms to anyone, there is plenty of evidence that across populations lead causes higher criminality, lower IQ, lower earnings, etc. And therefore, as a class, these people have been harmed by an actual dollar amount.