I guess USPS/FedEx/UPS drivers also have a lot of accumulated experience and 'tribal' knowledge that Amazon drivers lack.
Amazon also is still lacking the web access to manage the delivery that I get with UPS and FedEx.
UPS and Fedex pay drivers enough to care.
Amazon delivery drivers have no incentive, thus terrible service.
Disclaimer: when I have to order through Amazon, I have them flag my account to use traditional delivery methods because Amazon delivery is trash.