As a customer, I want a delivery time/date and let Amazon figure out how to do it. They're presumably way better at it than I am. If Amazon fails at that, I want to be able to complain without feeling like "oh, it was my fault because I picked Rickshaw Bob as the carrier".
When Amazon would willy-nilly give away a month of Prime when a guaranteed package was late, I would frequently wish I could get a package over to Mail Innovations or SurePost. Both would pretty reliably be late and for most packages, I'd rather have the $10 credit.