It's not about being the best, prime performance, or the minority of people doing the homework to notice they're ripped off. There's no retailer who isn't failing to give you the best price. A lot of shoppers don't even use prime, as long as they get that free delivery, they'll wait a few days. The majority of people are buying from them as they grow into a larger internet monopoly. They've been invested in online delivery for a long time, so strongly that even Walmart can't compete.

Out of all the retailers you listed, Walmart is the only close competitor in terms of delivery, Best Buy still has tech, but not much outside.