I do in fact think that if the FBI knew that certain people were using FedEx to reliably traffic in illegal substances inside sealed boxes, they would put pressure on FedEx to make it stop somehow, and raid them if they flatly refused. This might involve having them open the packages, but it more likely involves them identifying the people who these boxes come from and go to, and having them drop them as customers. And I am pretty confident that this does happen, because the profession of drug smuggling continues to exist, and druglords aren't just outsourcing their logistics to FedEx and UPS. I certainly agree that some fraction of their packages are illegal, but it's not straightforward to use the service this way.

Unlike UPS and FedEx, Tor makes this straightforward, and also tries to avoid knowing the identities of people who are sending things via their service. So the police don't have that option.

(Note that the above is simply a description of what Tor is currently doing, as I understand it, and not an attempt to make a moral judgment one way or the other as to what Tor should be doing.)