It isn't change for change's sake. User interface design has moved from being a kitchen sink to a more cleaner, taskcentric model. That affects the look and feel of UIs, the terminology, the flow between screens, the widgets and placement of buttons and other things. On top of that desktops must now function over a wider variety of displays and input devices including tablets and touch screens and at various screen densities and resolutions. Then there is accessibility which also introduces new input devices, readers, and other requirements. And IMEs for non-Western languages. And also changes to fonts and other metrics. And the technical debt of two maintaining and gluing entirely disparate visual experiences together.
Microsoft has perfectly valid reasons for changing the UI over time, but Windows 11 is the first OS I think where the coverage results in a decent, cohesive UI. Like I said I still hate the start menu but the desktop and control panel experience doesn't feel cobbled together from bits and pieces as it does in 10.