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This follows from long ongoing processes that aim to separate concerns, modularize, DRY, split into microservices etc, which has become even more important now that practically all computation is networked.

What you describe as an "app" is essentially just the front-end part, where the assumption is that some variation of components from a framework are sufficient to expose the essence (business logic) of the "full app" to the user.

Its possible that an application does not fit into this pattern. The richer the domain, the more unique or innovative the idea the more likely it may need its own core engine (see eg, gaming or scientific applications)