It’s not hard, but it’s still a hurdle. Warrants also can’t be requested from a judge by just anyone in the USPS iirc, so the start of the process often relies on an employee taking time out of their day to report something they deem suspicious in the first place, likely in an understaffed and overworked office that’s not built to handle the package volume of the area they serve.

Iirc, the USPS can’t unilaterally search your letters or packages because, as a government institution, it would be a major violation of the 4th amendment- even postal inspectors need to get a warrant to open a letter or package that’s not expressly addressed to them if it was in the care of the USPS. I think the only exception is when it’s an ‘Operation Santa’ letter, and there are regulations in place for how those get handled to protect the privacy of the sender.

The private parcel and package companies probably don’t have to abide by the same restrictions because they’re not government owned and operated.