But I can also fund a Robinhood account directly from my bank account through the iOS app and buy cryptocurrency. Or I can go on the ebay iOS app and buy a PDF

But I can also fund a Robinhood account directly from my bank account through the iOS app and buy cryptocurrency. Or I can go on the ebay iOS app and buy a PDF ebook from a seller. The mechanism of "in-app purchases" doesn't scale to ebay or Amazon or any other store where you might have millions of SKUs. Not to mention that you can't even buy Kindle books on the iOS app, even if you already have credit on Amazon (gift card, rewards points, etc.).