It's also a matter of incentives. Starbucks wants you in their app instead of as a widget in somebody else's - it lets them tell you about new products, cross-sell/up-sell, create habits, etc.

This general concept (embedding third parties as widgets in a larger product) has been tried many times before. Google themselves have done this - by my count - at least three separate times (Search, Maps, and Assistant).

None have been successful in large part because the third party being integrated benefits only marginally from such an integration. The amount of additional traffic these integrations drive generally isn't seen as being worth the loss of UX control and the intermediation in the customer relationship.