Well it would be nice if you could buy IAP from some other well-known merchants (for example, if you could pay MLB.tv directly to subscribe from the app rather than buying a subscription by paying Apple), but imagine the UX if every single app used their own store and you had to enter your card number for every different app, and all of the people demanding refunds from Apple when Apple had nothing to do with the purchase. Imagine how many people would give their credit card to an app thinking it was Apple they were trusting, then their card gets stolen.

It might be distasteful, but it makes sense from a risk avoidance standpoint