They had laws against killing people back then too. Why weren't those laws effective?

For example, the only reason the FDA had the power to go against Elixir sulfanilamide was that it was mislabeled, not that it was killing people.

You also have laws against copyright infringement. And just as you can't unsee an illegally downloaded movie, you can't unkill a man.

To an extent, you are right. On the other hand, those are usually (and correctly) regarded as government failures rather than as how things are supposed to work.

Reflexive secrecy is antithetical to democratic government, but that's a completely different issue.