The app model doesn't allow composability.

Apps are islands: little monoliths.

It's an extension of the web's same-origin model… but without the iframe composition primitive.

This is one of the reasons apps have a one-size-fits-all shape.

Other kinds of experiences (e.g. web apps) can be composed, but not safely… you either get a hellish privacy experience, or a confusing thicket of impossible-to-answer permission prompts.