But you need to go deeper. Amazon hasn't built apps that support those devices because those devices manufacturers want a cut of their profits off of subscriptions or rentals because those device manufacturers (Google / Apple) want people to use their own services instead.

That's only for in-app subscription purchases. You didn't think Apple was getting a 30% cut off Prime video subscriptions just because there's an iPad app, right? Or Netflix for that matter.

This is a problem entirely of Amazon's making. And I don't understand it. No one makes money on the devices, they make money on the subscriptions. It's in their best interests to be on as many devices as possible.

Or because they don’t support the specific DRM system used?