Stripe is only used for edge cases in Amazon's payment flow, and Amazon tracks how much of its payment flow and what the cost differential between them processing with their direct relationships would be versus Stripe's markup.
Basically, here are the worst of the worst of Amazon's transactions, please take this risk onto Stripe's platform, then we will cut cost with full knowledge of exactly how much we will save by spending developer resources to cut out Stripe for this particular card type in this particular country.
I would not call this a healthy relationship, but rather stripe taking the marginal additional volume that Amazon brings to their platform to net slightly more money and get slightly more data points on weird payment methods.