Chase or Citi or Wells Fargo or whoever is the bank that needs reserves and fronts the cash to the merchant, and earns on the interest rate and fees and takes the risk on defaults.
Even Amazon with all its projects has limited attention it can spend. Becoming a bank isn't something you half-ass, which is why they'll just continue to do cobranded cards with companies who are already banks.
Amazon needs credit cards more than the credit card networks need Amazon.
Also, Amazon transactions are virtually all card not present transactions.
But it's entirely possible that I'm reading the report wrong.
(Unless Amazon pays a higher fee but considers the 5% return to customers as some kind of "increase net spend on Amazon" benefit, but I think that's just Chase trying to kill Discover)
Also as of this year Costco lost me as a client since they dont accept Amex anymore. To me that was enough.