You can add repositories through GUI.

The reason installers are uncommon is a similar reason installers are uncommon on phones. Security and convenience. When your system is based around a unified “app store” system installers are generally not preferred because they bypass that and then the package manager can’t do anything about the program, and simply that the package manager provides a better experience overall.

Of course on Debian or Debian based distributions you can also download and install .deb files similarly to downloading a .exe installer on Windows, but that’s not preferred.

And developers of multiple DEs like Plasma, Gnome, etc, absolutely make it easy to use. I use Plasma, and it is incredibly intuitive.