Experience has taught us that the two operating modes are different enough by nature, that you need slightly different APIs for each (look at tools wrapping CLI programs: they generally suck one way or another).

What the industry settled on is building an API (library or service) around the data model and business logic, and then building UIs on top.

When done well, UIs are relatively shallow wrappers around the library, but do allow being creative about the UX too.