Sometimes it’s change for change’s sake. Any UI look and feel that’s been around for 10 years looks old (because it is), so you change it and now it’s new. And yeah the new look might not be as aesthetically pleasing, but it doesn’t look old and that’s good enough, and it too will be gone in a few years.
For me, a Unix user, the new look de jour is like the minimalism of the Bauhaus movement of the 1920s in architecture and reminds me of the old Athena X11 widget set of the 1980s and a return to the "twm" window manager.