Hopping in late to this thread and not a Pilot, however I work in the aviation field and have experience working with all the major US cargo operators-FedEx, UPS, Atlas, Amazon, Kalitta. In addition to carrying cargo versus people, you have to look at the operations. Cargo carriers normally are flying overnight. If you want to see it in action go to Memphis or Louisville at around 10pm and you will see a stack of planes heading in. Those planes are offloaded and reloaded in 4-6 hours before heading out again.
Second look at the fleets that they operate, generally older airframes. In fact up until last year FedEx was still flying the MD10-10 which were produced in 1970s. They still are operating the MD10-30 and MD11. That is not say they are not maintained well but it is to say they generally do not fly the latest and greatest airframes. The one exception is the 747-8F's and 777's that UPS and FedEx fly respectively.
For the large US cargo operators they each have a domestic hub-Memphis for FedEx, Louisville for UPS. Kallita's base is way up north in Oscoda, MI at an old AF base.