My reasoning is that it does one thing: Being a dumb head unit. Since it does just that one thing, it can be distributed as a big blob. You don't have to worry about changing that file or this file, following this instruction or that instruction, downloading this version or that version of the packages. It simplifies many things for the end user. I learned that from Retro Pie, a distro that I love.

Power users who want it to do many things can just run/read the scripts in the repo and make it a multi-purpose device. I'd imagine for $100 head unit, many people won't bother.

That being said, I really have to brush up on how to package Qt5 and OpenAuto as Debian packages.

Android Auto is more than just music/audio. It has on-screen navigation, and other driving- friendly features.

It looks like it's built on Rasbian with some binaries and systemd units, so it's not really a custom OS.

I second this, especially since I have been burned by single-purpose raspberry "distros" multiple times in the past in the audio sector. Maintaince is more work than people often realise.

Android auto is more than just music, there is navigation, video, and an abundance of apps.

"abundance" is a strong word, there are a "handful" at best.

The combination of navigation, audio, and Google Assistant is what I really like about AA.

Distro's are the new apps, bootable SD cards the new floppies.