Arbitrary Apple rules. Apple decided that "physical" goods & services in app purchases don't get taxed (e.g., clothes, uber rides). Digital goods do because they feel like they can get away with it which is why you can't buy eBooks from Kobo or Amazon on iOS, can't buy or rent movies from Google on iOS. People will tell you this is for "safety" and to keep you from getting scammed but there's nothing stopping malicious actors from creating apps that promise to ship physical goods and just don't. At least with digital goods you should get whatever you pay for immediately after purchase.
