Because most UNIX users see only this:

http://pubs.opengroup.org/onlinepubs/9699919799/

Everything else is fluff, and could stay just like it was on UNIX System V.

Hence why for macOS, iOS, tvOS, Android and ChromeOS, having an UNIX kernel is just a matter of convenience, given their application models and programming languages.

Apple and Google could eventually migrate to something else as kernel while keeping their Objective-C, Swift, Java and JavaScript APIs.

NeXT based NeXTStep on UNIX as a door into the then new UNIX worksations market, but it never had a CLI culture like the other UNIX vendors.

GUI workflows like on Xerox PARC were always important to Steve.