Isn't that how UPS and FedEx actually work? You can go to FedEx with an already-sealed box and pay them cash to deliver it. They don't know what it is, they're not going to open it, it could be anything. Presumably they have some kind of policy against illegal substances, but how are they supposed to verify that? Some of the packages they deliver most certainly are narcotics or child pornography or some other illegal thing.

You're saying that should justify the FBI going to all the FedEx hubs and opening all the packages to check for various illegal stuff?

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.)

Drug smuggling continues to exist because drug smuggling is wholesale and FedEx is retail. And FedEx wouldn't know to provide security to prevent competing dealers from holding up their drivers.

I suspect that package services are used with high frequency by criminals without these constraints, e.g. the ones committing mail fraud. The police deal with this not by searching the contents of random packages but by arresting the actual perpetrators regardless of what they use for a delivery service.

There seems to be a common impression that if the police can't find you using one specific investigative method then they have to give up and go home. There are hundreds of different ways to catch criminals. Most of the ones people complain about the police losing are ones they never traditionally had to begin with.