The idea is to maintain a consistent look (and feel) to lower the learning curve. As enforced by MacOS. 

''MacOS's contribution was supporting the Mac Toolbox which provided better and easier to use services than home grown re-invented alternatives.  But the important enforcement of consistent look and feel was the Apple Style Guide.  That lesson is being forgotten by MS, as they try to invent new ways to display and scroll. Most are a step backwards in usability.''--JimRussell