(Desktop) iTunes is the way it is in order to present a unified, familiar experience between the macOS and Windows iTunes desktop apps.
In Windows, iTunes is essentially "just enough macOS to manage an iOS device"—with every disparate responsibility that management entails all smushed together. (It's been this way since it was iPods that were being managed.) And macOS iTunes is the same codebase(!) as Windows iTunes, so it's not like that codebase is somehow going to compile as eight apps with their own UIs on macOS, vs. one on Windows. It's going to look, and work, on macOS as it does on Windows.