Basically, it’s really hard to actually develop totally separate UIs in the same app, e.g. another user brought up the Catalyst project, which is bringing poor-fit touch paradigms into macOS despite the developers’ intentions in many cases. One paradigm will be dominant. Care must also be taken to not load too many unused resources.

Interactive workflows also differ between UIs.

Since so much of an OS is the native UI and the first-party applications, even if the mobile, tablet and PC versions of an OS share some libraries, they can’t really share enough to meaningfully call them one OS without compromising the experience on all three.

So while there may be three pane email on the iPad as well as Mac, and email on iOS, they don’t really share enough to be called the same app, and if they did, at least one of them would suffer. And some of the interactions on the macOS version effectively can’t be brought over.