From starting up the session on the login manager to the final state of it there're various intermediate steps. Full desktop environments (GNOME and KDE) will l

From starting up the session on the login manager to the final state of it there're various intermediate steps. Full desktop environments (GNOME and KDE) will load everything needed for you whereas when running a window manager alone you've to do it yourself. Those Nix options set up things (essentially abstracting what you'll have done manually) in some previous step (before starting the window manager). >why does Ubuntu etc. make it dependent on the desktop session Because what if you didn't want them? Non-Nix systems don't have this centralized configuration model therefore they cannot provide this flexibility. Therefore they just let each session to load what it needs.