There are hundreds of devices out there capable of playing Amazon Video, they can't because Amazon either doesn't allow it (Android devices that aren't their branding) or charges a fee/device (ie. $1/Roku).

They want Vendor lock in, not in the sense that your data isn't portable, but that you rely on only their services. They do this by slowly pushing you onto more of their specific hardware and bundling more services essentially exclusively with their products