No one will know, and those that do will never reveal it. But I can surmise that it's a mutually net beneficial arrangement in the short-run, with Netflix expecting a better solution to present itself either internally or externally at some point. Otherwise, you're right, Netflix wouldn't use AWS and AWS would not supply to Netflix.

In a way, they're allocating money and resources to Amazon rather than developing internally simply because the long-term ROI doesn't warrant it.

And let's not discount the most obvious reason why it's probably not even a concern for Netflix: Amazon Prime Video sucks. Sure, Blockbuster probably said the same thing about Netflix at one point, but it's hindsight. I think most people, at the time, would have never guessed Netflix would be the one to pivot to online streaming either.

It's a very though provoking question and there's no way to distill it into one answer, so I'm just providing my POV.