Plus, the established couriers have several useful services that Amazon can't practically replicate. For instance, with UPS/FedEx/USPS I can easily divert all my packages to a nearby pickup point regularly or on short notice, even in a flyover suburb. Amazon will never be able to replicate that[1]. If they ever offer Amazon Flex in my area, I hope there will be a way to opt out.

[1] I'm aware of Amazon Locker, but I don't live in Seattle, the Bay Area, or Manhattan; and dealing with yet another pickup point for a specific merchant just seems like an annoying hassle.