

They did that for Ubuntu. I mean it makes sense on Ubuntu. For everything else you can install it through Flatpak, your distributions own package manager (but that is often not the newest one) or AppImage, through Steam or many other methods. Its amazing how many ways you can install this and where it is available on!
I started with RetroPie long time ago too. :-) RetroPie is an operating system that is build to be a Gaming distribution basically. It uses RetroArch on its backend for the emulators and Emulation Station for the UI. When you select and run a game in Emulation Station (the UI on operating system level), then it runs RetroArch with a core and a game. While ingame, you can open the RetroArch menu as well.
In short: RetroPie is an operating system setup to use RetroArch for the emulation.