Because apps can’t link to purchase outside the App Store. Let alone link, they can’t even mention they can purchase the content elsewhere either with images, text or any form.
I think this is so silly on Apple side as they try to have a name on raising the bar for user experience. And the worse is that before I realized what was going on I thought it was Amazon fault for some reason, which is even better for Apple as people would normally blame the developers rather than the platform for this design decision.