Not a UI expert professionally but here's what it is: Windows has standardized and reused GUI elements across every application. So it's immediately clear what

Not a UI expert professionally but here's what it is: Windows has standardized and reused GUI elements across every application. So it's immediately clear what's a button, tabs, a subwindow, and how tab hierarchies work (notice they have a bounding box on the tab contents they will change). Modern web-design disregards all of this because it prioritizes being visually pretty over being easily-recognizable.