Currently for FedEx Home Delivery, which is most similar to Amazon Delivery, FedEx sells delivery routes to independent delivery operators which they then pay to deliver packages on those routes. The delivery operators then employee the drivers and not FedEx. In many circumstances the independent operators and delivery drivers are one and the same. The drivers tried to unionize saying they were in reality employees of FedEx. FedEx refused to recognize the union and a claim was filed with National Labor Relations Board. The NLRB sided with the drivers but their decision was overruled by the DC circuit court of appeals in 2017. By that time a new administration was in charge and the decision of the court was not appealed by the NLRB. The court based their decision by the fact that the independent delivery operators can hire/fire drivers and helpers without input from FedEx and that they can resell the routes independently of FedEx.
Amazon Delivery operates under a similar model. All those Amazon delivery vans you see are run by independent delivery operators which hire the drivers. Each delivery operator runs at least 5 vans with the norm being between 20 - 40.
Amazon simply reached a point where they would have had to co invest woth their carriers. So investing in oen assets made a lot more sense. And Amazon has the scale, and means to control their sypply chain, to maximize utilization of their transpirt assets.
Worth noting so, that Amazon is using a lot of third party drivers, same as DHL and others are doing.