Growing up in Wisconsin I can tell you there is a large business of shipping bodies from AZ and FL back to WI.

Let's say a person dies in FL and the family wants a burial back in WI. An undertaker in FL would prepare the body, then arrange to transport the body to the airport cargo area. When the plane lands in WI, a undertaker in WI (who would be coordinating with the family and the undertaker in FL) would be there with the funeral home's hearse.

The business of shipping bodies is lucrative enough that some airlines offer Frequent Flyer miles to undertakers to use them to ship coffins.

When I worked in VA I visited the UA cargo building at IAD quite often; it was not uncommon at all to see a metal transport coffin coming or going.