TLDR (for my comment) : Amazon are building their own UPS because they can. Any othe arguments in favor of it be it efficiency or costs are secondary.
Longer version:
Not sure how Amazon deliveries by UPS are in the US but in the U.K. there are carriers which offered far better delivery service than what is known as Amazon Logistics here in the U.K. Once Amazon started using Amazon Logistics, the level of service dropped significantly. False deivery attempts were common and Prime Next Day were not happening next day 3 out of 5 times. I cancelled my Prime membership.
After about a couple of years I have resubscribed to Prime since it is now much better value AND Amazon Logistics have noticably improved. Next day and even Same day deliveries are indeed happening.
They didn't need to setup Amazon Logistics and suffer poor service quality. There were near perfect couriers (for e.g. DPD they even offer tracking your courier driver on a map in near real time).
Longer version: Not sure how Amazon deliveries by UPS are in the US but in the U.K. there are carriers which offered far better delivery service than what is known as Amazon Logistics here in the U.K. Once Amazon started using Amazon Logistics, the level of service dropped significantly. False deivery attempts were common and Prime Next Day were not happening next day 3 out of 5 times. I cancelled my Prime membership.
After about a couple of years I have resubscribed to Prime since it is now much better value AND Amazon Logistics have noticably improved. Next day and even Same day deliveries are indeed happening.
They didn't need to setup Amazon Logistics and suffer poor service quality. There were near perfect couriers (for e.g. DPD they even offer tracking your courier driver on a map in near real time).