Some comments have mentioned "What would be the advantage of buying Stripe?" For a company like Amazon, a buyout of Stripe mitigates risk. As the Robber Baron

Some comments have mentioned "What would be the advantage of buying Stripe?" For a company like Amazon, a buyout of Stripe mitigates risk. As the Robber Barons said, "Don't let anyone get between you and your customer." If Amazon has indeed done an evaluation that Strip is more cost-efficient, secure, flexible, or whatever, than maintaining their own payment processing, they don't want to be in a position of Stripe strong-arming them down the road.