They are targeting users that don't want to install their packages or learn how to set up a new wm or theme. That's simply it. As much as distros and the Linux ecosystem prides itself on its customizability, there will always be people (myself included) that simply want an easy way to run something with preinstalled defaults that matches my aesthetic and functional requirements. Yes, I could install Ubuntu, and then go ahead and install some other packages and a theme. This saves me several steps and they'll presumably keep it updated and compatible with each other nicely in a way that I couldn't care to as an end user with my modified Ubuntu.