I do it with a few text files.
> sudo darch images pull pauldotknopf/darch-ubuntu-$IMAGE
So your "build" is actually just grabbing what the distro already built and then using its infrastructure.
Your description sounded more like you were actually doing things from scratch or at least weren't relying on a distro.
There has been quite a bit of standardization and improved compatibility, so that you can choose relatively freely between the "big" distros. But that does not say they are a commodity but rather that all (five or so) of them are good.