> This rings true to me as well, but why is this the case?

Because the Apple macOS ecosystem already has an coherent design at the point where I (as a macOS user) would hate to bring another app that breaks the coherence.

Also Apple's official apps are generally much better than the MS ones or Gnome ones (see IE vs Safari) which makes the expectation of users higher.

IMO Apple really made a great, healthy ecosystem around the macOS.