80s 90s and early 2000s it was the OS. All apps running under a specific OS would look similar to each other. Behave same way. File menus looked same. Preferences was in same location.
But somewhere in 2000s this reversed and OS was no longer driver of consistency.
App developers stopped following OS UI guidelines and decided that it's not important for OS to be cohesive. The goal was that app, no matter where it is running, should have consistent look and behavior.
And now we are in application centric world where application dictates UI and there is no OS level consistency.
Additionally Microsoft and Apple did not help the situation by themselves breaking the OS UI paradigms with media players, or in MS case MS Office.