The whole galaxy of Prime services surrounding package delivery would never survive on its own -- or, another way to see it is that they don't have the market pressure necessary to improve/get fixed due to how they are tied to the package delivery.

Prime music for the reason you give, Prime video who keeps finding way to suggest you movies you need to pay extra for (you go specifically to "free/included", you click a couple times, and you're being recommended extra costs things ...), Amazon Drive keeps your file "forever" but then no sorry we're deleting them we stop this but then wait no we're making Amazon Photos now your photos will be kept everything else deleted but wait no only some photos will be transfered ...

The people managing those services are coasting on other people's success.